Application software (app for short) is software designed to perform a group of coordinated functions, tasks, or activities for the benefit of the user. Examples of an application include a word processor, a spreadsheet, an accounting application, a web browser, an email client, a media player, a file viewer, an aeronautical flight simulator, a console game or a photo editor. The collective noun application software refers to all applications collectively.^[1] This contrasts with system software, which is mainly involved with running the computer.

Applications may be bundled with the computer and its system software or published separately, and may be coded as proprietary, open-source or university projects.^[2] Apps built for mobile platforms are called mobile apps.

In information technology, an application (app), application program or software application is a computer program designed to help people perform an activity. An application thus differs from an operating system (which runs a computer), a utility (which performs maintenance or general-purpose chores), and a programming tool (with which computer programs are created). Depending on the activity for which it was designed, an application can manipulate text, numbers, audio, graphics and a combination of these elements. Some application packages focus on a single task, such as word processing; others, called integrated software include several applications.^[3]

User-written software tailors systems to meet the user's specific needs. User-written software includes spreadsheet templates, word processor macros, scientific simulations, audio, graphics, and animation scripts. Even email filters are a kind of user software. Users create this software themselves and often overlook how important it is.

The delineation between system software such as operating systems and application software is not exact, however, and is occasionally the object of controversy.^[4] For example, one of the key questions in the United States v. Microsoft Corp. antitrust trial was whether Microsoft's Internet Explorer web browser was part of its Windows operating system or a separable piece of application software. As another example, the GNU/Linux naming controversy is, in part, due to disagreement about the relationship between the Linux kernel and the operating systems built over this kernel. In some types of embedded systems, the application software and the operating system software may be indistinguishable to the user, as in the case of software used to control a VCR, DVD player or microwave oven. The above definitions may exclude some applications that may exist on some computers in large organizations. For an alternative definition of an app: see Application Portfolio Management.

The word "application", once used as an adjective, is not restricted to the "of or pertaining to application software" meaning.^[5] For example, concepts such as application programming interface (API), application server, application virtualization, application lifecycle management and portable application apply to all computer programs alike, not just application software.

Some applications are available in versions for several different platforms; others only work on one and are thus called, for example, a Geography application for Microsoft Windows, or an Android application for education, or a Linux game. Sometimes a new and popular application arises which only runs on one platform, increasing the desirability of that platform. This is called a killer application or killer app. For example, VisiCalc was the first modern spreadsheet software for the Apple II and helped selling the then-new personal computers into offices. For Blackberry it was their email software.

In recent years, the shortened term "app" (coined in 1981 or earlier^[6]) has become popular to refer to applications for mobile devices such as smartphones and tablets, the shortened form matching their typically smaller scope compared to applications on PCs. Even more recently, the shortened version is used for desktop application software as well.

There are many different and alternative ways in order to classify application software.

By the legal point of view, application software is mainly classified with a black box approach, in relation to the rights of its final end-users or subscribers (with eventual intermediate and tiered subscription levels).

Software applications are also classified in respect of the programming language in which the source code is written or executed, and respect of their purpose and outputs.

Application software is usually distinguished among two main classes: closed source vs open source software applications, and among free or proprietary software applications.

Proprietary software is placed under the exclusive copyright, and a software license grants limited usage rights. The open-closed principle states that software may be "open only for extension, but not for modification". Such applications can only get add-on by third-parties.

Free and open-source software shall be run, distributed, sold or extended for any purpose, and -being open- shall be modified or reversed in the same way.

FOSS software applications released under a free license may be perpetual and also royalty-free. Perhaps, the owner, the holder or third-party enforcer of any right (copyright, trademark, patent, or ius in re aliena) are entitled to add exceptions, limitations, time decays or expiring dates to the license terms of use.

Public-domain software is a type of FOSS, which is royalty-free and - openly or reservedly- can be run, distributed, modified, reversed, republished or created in derivative works without any copyright attribution and therefore revocation. It can even be sold, but without transferring the public domain property to other single subjects. Public-domain SW can be released under an (un)licens ing legal statement, which enforces those terms and conditions for an indefinite duration (for a lifetime, or forever).

Since the development and near-universal adoption of the web, an important distinction that has emerged, has been between web applications — written with HTML, JavaScript and other web-native technologies and typically requiring one to be online and running a web browser, and the more traditional native applications written in whatever languages are available for one's particular type of computer. There has been a contentious debate in the computing community regarding web applications replacing native applications for many purposes, especially on mobile devices such as smartphones and tablets. Web apps have indeed greatly increased in popularity for some uses, but the advantages of applications make them unlikely to disappear soon, if ever. Furthermore, the two can be complementary, and even integrated.^[7][8][9]

Application software can also be seen as being either horizontal or vertical.^[10][11] Horizontal applications are more popular and widespread, because they are general purpose, for example word processors or databases. Vertical applications are niche products, designed for a particular type of industry or business, or department within an organization. Integrated suites of software will try to handle every specific aspect possible of, for example, manufacturing or banking worker, or accounting, or customer service.

There are many types of application software:^[12]

Applications can also be classified by computing platform such as a particular operating system, delivery network such as in cloud computing and Web 2.0 applications, or delivery devices such as mobile apps for mobile devices.

The operating system itself can be considered application software when performing simple calculating, measuring, rendering, and word processing tasks not used to control hardware via command-line interface or graphical user interface. This does not include application software bundled within operating systems such as a software calculator or text editor.