WordPress is a free and open-source content management system (CMS) based on PHP and MySQL.  To function, WordPress has to be installed on a web server, which would either be part of an Internet hosting service or a network host in its own right. An example of the first scenario may be a service like WordPress.com, for example, and the second case could be a computer running the software package WordPress.org.  A local computer may be used for single-user testing and learning purposes. Features include a plugin architecture and a template system. WordPress was used by more than 27.5% of the top 10 million websites as of February 2017.  WordPress is reportedly the most popular website management or blogging system in use on the Web,  supporting more than 60 million websites.  WordPress has also been used for other application domains such as pervasive display systems (PDS) .
WordPress has a web template system using a template processor. Its architecture is a front controller, routing all requests for non-static URI s to a single PHP file which parses the URI and identifies the target page. This allows support for more human-readable permalinks. 
WordPress users may install and switch among different themes. Themes allow users to change the look and functionality of a WordPress website without altering the core code or site content. Every WordPress website requires at least one theme to be present and every theme should be designed using WordPress standards with structured PHP, valid HTML (HyperText Markup Language), and Cascading Style Sheets (CSS). Themes may be directly installed using the WordPress "Appearance" administration tool in the dashboard, or theme folders may be copied directly into the themes directory, for example via FTP. The PHP, HTML and CSS found in themes can be directly modified to alter theme behavior, or a theme can be a "child" theme which inherits settings from another theme and selectively overrides features.  WordPress themes are generally classified into two categories: free and premium. Many free themes are listed in the WordPress theme directory, and premium themes are available for purchase from marketplaces and individual WordPress developers. WordPress users may also create and develop their own custom themes. The free theme Underscores created by the WordPress developers has become a popular basis for new themes. 
WordPress' plugin architecture allows users to extend the features and functionality of a website or blog. WordPress has over 50,316 plugins available,  each of which offers custom functions and features enabling users to tailor their sites to their specific needs. These customizations range from search engine optimization, to client portals used to display private information to logged in users, to content management systems, to content displaying features, such as the addition of widgets and navigation bars. Not all available plugins are always abreast with the upgrades and as a result they may not function properly or may not function at all.  Most plugins are available through WordPress themselves, either via downloading them and installing the files manually via FTP or through the WordPress dashboard. However, many third parties offer plugins through their own websites, many of which are paid packages.
Native applications exist for WebOS,  Android,  iOS (iPhone, iPod Touch, iPad),   Windows Phone, and BlackBerry.  These applications, designed by Automattic, have options such as adding new blog posts and pages, commenting, moderating comments, replying to comments in addition to the ability to view the stats.  
WordPress also features integrated link management; a search engine –friendly, clean permalink structure; the ability to assign multiple categories to posts; and support for tagging of posts. Automatic filters are also included, providing standardized formatting and styling of text in posts (for example, converting regular quotes to smart quotes). WordPress also supports the Trackback and Pingback standards for displaying links to other sites that have themselves linked to a post or an article. WordPress posts can be edited in HTML, using the visual editor, or using one of a number of plugins that allow for a variety of customized editing features.
Multi-user and multi-blogging
Prior to version 3, WordPress supported one blog per installation, although multiple concurrent copies may be run from different directories if configured to use separate database tables. WordPress Multisites  (previously referred to as WordPress Multi-User, WordPress MU, or WPMU) was a fork of WordPress created to allow multiple blogs to exist within one installation but is able to be administered by a centralized maintainer. WordPress MU makes it possible for those with websites to host their own blogging communities, as well as control and moderate all the blogs from a single dashboard. WordPress MS adds eight new data tables for each blog.
As of the release of WordPress 3, WordPress MU has merged with WordPress. 
b2/cafelog, more commonly known as b2 or cafelog, was the precursor to WordPress.  b2/cafelog was estimated to have been installed on approximately 2,000 blogs as of May 2003.  It was written in PHP for use with MySQL by Michel Valdrighi, who is now a contributing developer to WordPress. Although WordPress is the official successor, another project, b2evolution, is also in active development.
WordPress first appeared in 2003 as a joint effort between Matt Mullenweg and Mike Little to create a fork of b2.  Christine Selleck Tremoulet, a friend of Mullenweg, suggested the name WordPress.  
In 2004 the licensing terms for the competing Movable Type package were changed by Six Apart, resulting in many of its most influential users migrating to WordPress.   By October 2009 the Open Source CMS MarketShare Report concluded that WordPress enjoyed the greatest brand strength of any open-source content management system.
As of February 2017, WordPress is used by 58.7% of all the websites whose content management system is known. This is 27.5% of the top 10 million websites. 
|Legend:||Old version||Older version, still supported||Current stable version||Latest preview version||Future release|
|Version||Code name||Release date||Notes|
|Old version, no longer supported: 0.7||none||May 27, 2003 ||Used the same file structure as its predecessor, b2/cafelog, and continued the numbering from its last release, 0.6.  Only 0.71-gold is available for download in the official WordPress Release Archive page.|
|Old version, no longer supported: 1.0||Davis||January 3, 2004 ||Added search engine friendly permalinks, multiple categories, dead simple installation and upgrade, comment moderation, XFN support, Atom support.|
|Old version, no longer supported: 1.2||Mingus||May 22, 2004 ||Added support of Plugins; which same identification headers are used unchanged in WordPress releases as of 2011.|
|Old version, no longer supported: 1.5||Strayhorn||February 17, 2005 ||Added a range of vital features, such as ability to manage static pages and a template/Theme system. It was also equipped with a new default template (code named Kubrick ).  designed by Michael Heilemann.|
|Old version, no longer supported: 2.0||Duke||December 31, 2005 ||Added rich editing, better administration tools, image uploading, faster posting, improved import system, fully overhauled the back end, and various improvements to Plugin developers.|
|Old version, no longer supported: 2.1||Ella||January 22, 2007 ||Corrected security issues, redesigned interface, enhanced editing tools (including integrated spell check and auto save), and improved content management options.|
|Old version, no longer supported: 2.2||Getz||May 16, 2007 ||Added widget support for templates, updated Atom feed support, and speed optimizations.|
|Old version, no longer supported: 2.3||Dexter||September 24, 2007 ||Added native tagging support, new taxonomy system for categories, and easy notification of updates, fully supports Atom 1.0, with the publishing protocol, and some much needed security fixes.|
|Old version, no longer supported: 2.5||Brecker||March 29, 2008 ||Major revamp to the dashboard, dashboard widgets, multi-file upload, extended search, improved editor, improved plugin system and more.|
|Old version, no longer supported: 2.6||Tyner||July 15, 2008 ||Added new features that made WordPress a more powerful CMS: it can now track changes to every post and page and allow easy posting from anywhere on the web.|
|Old version, no longer supported: 2.7||Coltrane||December 11, 2008 ||Administration interface redesigned fully, added automatic upgrades and installing plugins, from within the administration interface.|
|Old version, no longer supported: 2.8||Baker||June 10, 2009 ||Added improvements in speed, automatic installing of themes from within administration interface, introduces the CodePress editor for syntax highlighting and a redesigned widget interface.|
|Old version, no longer supported: 2.9||Carmen||December 19, 2009 ||Added global undo, built-in image editor, batch plugin updating, and many less visible tweaks.|
|Old version, no longer supported: 3.0||Thelonious||June 17, 2010 ||Added a new theme APIs, merge WordPress and WordPress MU, creating the new multi-site functionality, new default theme "Twenty Ten" and a refreshed, lighter admin UI.|
|Old version, no longer supported: 3.1||Reinhardt||February 23, 2011 ||Added the Admin Bar, which is displayed on all blog pages when an admin is logged in, and Post Format, best explained as a Tumblr like micro-blogging feature. It provides easy access to many critical functions, such as comments and updates. Includes internal linking abilities, a newly streamlined writing interface, and many other changes.|
|Old version, no longer supported: 3.2||Gershwin||July 4, 2011 ||Focused on making WordPress faster and lighter. Released only four months after version 3.1, reflecting the growing speed of development in the WordPress community.|
|Old version, no longer supported: 3.3||Sonny||December 12, 2011 ||Focused on making WordPress friendlier for beginners and tablet computer users.|
|Old version, no longer supported: 3.4||Green||June 13, 2012 ||Focused on improvements to theme customization, Twitter integration and several minor changes.|
|Old version, no longer supported: 3.5||Elvin||December 11, 2012 ||Support for the Retina Display, color picker, new default theme "Twenty Twelve", improved image workflow.|
|Old version, no longer supported: 3.6||Oscar||August 1, 2013 ||New default theme "Twenty Thirteen", admin enhancements, post formats UI update, menus UI improvements, new revision system, autosave and post locking.|
|Older version, yet still supported: 3.7||Basie||October 24, 2013 ||Automatically apply maintenance and security updates in the background, stronger password recommendations, support for automatically installing the right language files and keeping them up to date.|
|Older version, yet still supported: 3.8||Parker||December 12, 2013 ||Improved admin interface, responsive design for mobile devices, new typography using Open Sans, admin color schemes, redesigned theme management interface, simplified main dashboard, "Twenty Fourteen" magazine style default theme, second release using "Plugin-first development process".|
|Older version, yet still supported: 3.9||Smith||April 16, 2014 ||Improvements to editor for media, live widget and header previews, new theme browser.|
|Older version, yet still supported: 4.0||Benny||September 4, 2014 ||Improved media management, embeds, writing interface, easy language change, theme customizer, plugin discovery and compatibility with PHP 5.5 and MySQL 5.6. |
|Older version, yet still supported: 4.1||Dinah||December 18, 2014 ||Twenty Fifteen as the new default theme, distraction-free writing, easy language switch, Vine embeds and plugin recommendations.|
|Older version, yet still supported: 4.2||Powell||April 23, 2015 ||New "Press This" features, improved characters support, emoji support, improved customizer, new embeds and updated plugin system.|
|Older version, yet still supported: 4.3||Billie||August 18, 2015 ||Focus on mobile experience, better passwords and improved customizer.|
|Older version, yet still supported: 4.4||Clifford||December 8, 2015 ||Introduction of "Twenty Sixteen" theme, and improved responsive images and embeds.|
|Older version, yet still supported: 4.5||Coleman||April 12, 2016 ||Added inline linking, formatting shortcuts, live responsive previews, and other updates under the hood.|
|Older version, yet still supported: 4.6||Pepper||August 16, 2016 ||Added streamlined updates, native fonts, editor improvements with inline link checker and content recovery, and other updates under the hood.|
|Older version, yet still supported: 4.7||Vaughan||December 6, 2016 ||Comes with new default theme "Twenty Seventeen", Video Header Support, PDF preview, custom CSS in live preview, editor Improvements, and other updates under the hood.|
|Older version, yet still supported: 4.8||Evans||June 8, 2017 ||The next-generation editor. Additional specific goals include the TinyMCE inline element / link boundaries, new media widgets, WYSIWYG in text widget. End Support for Internet Explorer Versions 8, 9, and 10.|
|Current stable version: 4.9||Tipton||November 16, 2017 ||Improved theme customizer experience, including scheduling, frontend preview links, autosave revisions, theme browsing, improved menu functions, and syntax highlighting. Added new gallery widget and updated text and video widgets. Theme editor gives warnings and rollbacks when saving files that produce fatal errors. |
|Future release: 5.0||TBD||2018||WordPress 5.0 will be the first “major” release of 2018, including the new editor, codenamed “Gutenberg”.|
Many security issues  have been uncovered in the software, particularly in 2007, 2008, and 2015. According to Secunia, WordPress in April 2009 had seven unpatched security advisories (out of 32 total), with a maximum rating of "Less Critical". Secunia maintains an up-to-date list of WordPress vulnerabilities. 
In January 2007, many high-profile search engine optimization (SEO) blogs, as well as many low-profile commercial blogs featuring AdSense, were targeted and attacked with a WordPress exploit.  A separate vulnerability on one of the project site's web servers allowed an attacker to introduce exploitable code in the form of a back door to some downloads of WordPress 2.1.1. The 2.1.2 release addressed this issue; an advisory released at the time advised all users to upgrade immediately. 
In May 2007, a study revealed that 98% of WordPress blogs being run were exploitable because they were running outdated and unsupported versions of the software.  In part to mitigate this problem, WordPress made updating the software a much easier, "one click" automated process in version 2.7 (released in December 2008).  However, the filesystem security settings required to enable the update process can be an additional risk. 
In a June 2007 interview, Stefan Esser, the founder of the PHP Security Response Team, spoke critically of WordPress' security track record, citing problems with the application's architecture that made it unnecessarily difficult to write code that is secure from SQL injection vulnerabilities, as well as some other problems. 
In June 2013, it was found that some of the 50 most downloaded WordPress plugins were vulnerable to common Web attacks such as SQL injection and XSS. A separate inspection of the top-10 e-commerce plugins showed that seven of them were vulnerable. 
In an effort to promote better security, and to streamline the update experience overall, automatic background updates were introduced in WordPress 3.7. 
Individual installations of WordPress can be protected with security plugins that prevent user enumeration, hide resources and thwart probes. Users can also protect their WordPress installations by taking steps such as keeping all WordPress installation, themes, and plugins updated, using only trusted themes and plugins,  editing the site's .htaccess file to prevent many types of SQL injection attacks and block unauthorized access to sensitive files. It is especially important to keep WordPress plugins updated because would-be hackers can easily list all the plugins a site uses, and then run scans searching for any vulnerabilities against those plugins. If vulnerabilities are found, they may be exploited to allow hackers to upload their own files (such as a PHP Shell script) that collect sensitive information.   
Developers can also use tools to analyze potential vulnerabilities, including WPScan, WordPress Auditor and WordPress Sploit Framework developed by 0pc0deFR. These types of tools research known vulnerabilities, such as a CSRF, LFI, RFI, XSS, SQL injection and user enumeration. However, not all vulnerabilities can be detected by tools, so it is advisable to check the code of plugins, themes and other add-ins from other developers.
In March 2015, it was reported by many security experts and SEOs including Search Engine Land that a SEO plugin for WordPress called Yoast which is used by more than 14 million users worldwide has a vulnerability which can lead to an exploit where hackers can do a Blind SQL injection.   To fix that issue they immediately introduced a newer version 1.7.4 of the same plugin to avoid any disturbance on web because of the security lapse that the plugin had. 
In January 2017, security auditors at Securi identified a vulnerability in the WordPress REST API that would allow any unauthenticated user to modify any post or page within a site running WordPress 4.7 or greater. The auditors quietly notified WordPress developers, and within six days WordPress released a high priority patch to version 4.7.2 which addressed the problem.  
WordPress' minimum PHP version requirement is PHP 5.2,  which was released on January 6, 2006,  11 years ago, and which has been unsupported by the PHP Group and not received any security patches since January 6, 2011, 6 years ago. 
Development and support
WordPress is also developed by its community, including WP testers, a group of volunteers who test each release.  They have early access to nightly builds, beta versions and release candidates. Errors are documented in a special mailing list, or the project's Trac tool.
Though largely developed by the community surrounding it, WordPress is closely associated with Automattic, the company founded by Matt Mullenweg. On September 9, 2010, Automattic handed the WordPress trademark to the newly created WordPress Foundation, which is an umbrella organization supporting WordPress.org (including the software and archives for plugins and themes), bbPress and BuddyPress.
WordCamp developer and user conferences
WordCamps are casual, locally organized conferences covering everything related to WordPress.  The first such event was WordCamp 2006 in August 2006 in San Francisco, which lasted one day and had over 500 attendees.   The first WordCamp outside San Francisco was held in Beijing in September 2007.  Since then, there have been over 507 WordCamps in over 207 cities in 48 different countries around the world.  WordCamp San Francisco 2014 was the last official annual conference of WordPress developers and users taking place in San Francisco, having now been replaced with WordCamp US. 
WordPress' primary support website is WordPress.org. This support website hosts both WordPress Codex, the online manual for WordPress and a living repository for WordPress information and documentation, and WordPress Forums, an active online community of WordPress users.