Y Combinator is an American seed accelerator, started in March 2005. Fast Company has called YC "the world's most powerful start-up incubator".  Fortune has called Y Combinator "a spawning ground for emerging tech giants". 
In its main program, Y Combinator interviews and selects two batches of companies per year. The companies receive seed money, advice, and connections in exchange for 7% equity.  The program includes "office hours", where startup founders meet individually and in groups with Y Combinator partners for advice. Founders also participate in weekly dinners where guests from the Silicon Valley ecosystem (successful entrepreneurs, venture capitalists, etc.) speak to the founders.
Y Combinator’s motto is “Make Something People Want.”  The program aims to focus the founders on further developing their product, team and market, refining their business model, achieving product/market fit, and scaling the startup into a high growth business, etc. The program culminates at Demo Day where startups present their business to a selected audience of investors. 
As of 2016, Y Combinator had invested in ~940 companies including Dropbox, Airbnb, Coinbase, Stripe, Reddit, Zenefits, BuildZoom, Instacart, Twitch.tv, Machine Zone, Weebly, Paribus, Chinese startup Raven Tech,  and the combined market capitalization of YC companies was over $65B. 
Non-profit organizations can also participate in the main YC program. 
In 2015 YC introduced additional programs:
- In July 2015, Y Combinator introduced the YC Fellowship Program, aimed at companies at an earlier stage than the main program. 
- In October 2015, Y Combinator introduced the YC Continuity Fund. The fund allows Y Combinator to make pro rata investments in their alumni companies with valuations under $300 million. Y Combinator will also consider leading or participating in later stage growth financing rounds for YC companies. 
- In October 2015 YC introduced YC Research, to fund long-term fundamental research. YC President Sam Altman donated $10m. 
Y Combinator was started in 2005 by Paul Graham, Jessica Livingston, Trevor Blackwell and Robert Tappan Morris.  From 2005 to 2008 one program was held in Cambridge, Massachusetts, and one was held in Mountain View, California. In January 2009, Y Combinator announced that the Cambridge program would be closed and all future programs would take place in Silicon Valley. 
In 2009, Sequoia Capital led the $2 million investment round into an entity of Y Combinator which would allow the company to invest in approximately 60 companies a year as opposed to their previous 40 companies a year.  The following year, Sequoia led a $8.25 million funding round for Y Combinator to further increase the number of startups the company could fund. 
Then in 2011, Yuri Milner and SV Angel offered every Y Combinator company a $150,000 convertible note investment.  The amount put in to each company was changed to $80,000 when Start Fund was renewed. 
In September 2013, Paul Graham announced Y Combinator would fund nonprofit organizations accepted into its program after having tested the concept with Watsi (while continuing to fund mostly for-profit startups). 
In 2014, founder Paul Graham announced he was stepping down and that Sam Altman would take over as President of Y Combinator.  That same year, Altman announced "The New Deal" for YC startups, which offers $120,000 for 7% equity. 
Late in 2014, Sam Altman announced a partnership with Transcriptic to provide increased support for Y Combinator's growing community of biotech companies.  Then in 2015, he announced a partnership with Bolt and increased support for hardware companies. 
On 11 August 2016, YC announced that YC partners will be visiting 11 countries this fall to meet with founders and learn more about how we can be helpful to international startup communities. These 11 countries are Nigeria, Denmark, Portugal, Sweden, Germany, Russia, Argentina, Chile, Mexico, Israel, and India.
In September 2016, Y Combinator announced shuffling the deck at the Mountain View startup accelerator again, with Altman announcing that he will now be president of YC Group, which includes Y Combinator, the YC Continuity fund that was launched last October and the YC Research "moon shot" program. Ali Rowghani, Twitter's former chief financial officer and chief operating officer who was put in charge of the YC Continuity Fund when it started, is now CEO of YC Continuity. Michael Seibel, who co-founded Justin.tv, is the new CEO of YC Core, the program that Paul Buchheit has run since earlier this year. 
As of November 2016, Michael Seibel serves as the Chief Executive Officer at Y Combinator. He is the first African-American partner at Y-Combinator and his primary role is to increase minority representation and diversity for the incubator. 
In early 2010, Harj Taggar, cofounder of Y Combinator-funded Auctomatic, joined as an advisor. In September 2010, Alexis Ohanian, co-founder of Y Combinator-backed Reddit, joined.  In November 2010, Gmail creator Paul Buchheit and Harj Taggar were named partners.  In 2015, Taggar left YC to start Triplebyte, a startup aiming to assist companies with their technical hiring needs. 
In the summer of 2014, Sam Altman became president of Y Combinator.  Y Combinator also announced a Board of Overseers: Brian Chesky, cofounder of AirBnB, Adora Cheung, cofounder of Homejoy, Patrick Collison, cofounder of Stripe, Drew Houston, founder of Dropbox, Jessica Livingston, David Rusenko, Emmett Shear, and Sam Altman, cofounder of Loopt.
In January 2015 it was announced that Paul Buchheit would be named managing partner for Y Combinator's core program and Kevin Hale would be the managing partner for the Fellowship. 
As of February 2015, additional Y Combinator partners are Dalton Caldwell, founder of imeem and app.net ; Jared Friedman, founder of Scribd ; Kevin Hale, cofounder of Wufoo ; Aaron Harris, cofounder of Tutorspree; Justin Kan, cofounder of Exec, Twitch.tv and Justin.tv ; Attorneys Carolynn Levy and Jon Levy; Kat Manalac; Kirsty Nathoo; Geoff Ralston, creator of Rocketmail ; Michael Seibel, cofounder of Socialcam ; and Qasar Younis, cofounder of Talkbin. Ali Rowghani is the managing partner of YC Continuity. 
In 2013, Y Combinator began accepting nonprofit organizations. Notable Y Combinator-backed nonprofits include Watsi (crowdfunding medical treatment in developing countries), Immunity Project (using machine learning to develop an HIV/AIDS vaccine), Noora Health (empowering patient family caregivers to take better care of their loved ones at home), CareMessage (using mobile technology to help improve health outcomes for underserved patients) and Zidisha (direct person-to-person lending to developing countries). 
The YC Fellowship Program was announced in July 2015, with the goal of funding companies at the idea or prototype stage.  The first batch of YC Fellowship included 32 companies that received an equity-free grant instead of an investment. 
In January 2016 Y Combinator announced version 2 of the program, with participating companies receiving $20k investment for an 1.5% equity stake. The equity stake is structured as a convertible security that only converts into shares if a company has an IPO, or a funding event or acquisition that values the company at $100m or more. 
YC Research was announced in October 2015. It is a nonprofit research lab focused on work that requires a very long time horizon, seeks to answer very open-ended questions, or develops technology that shouldn’t be owned by any one company. Researchers are paid as full-time employees and can receive equity in Y Combinator.    OpenAI was the first project undertaken by YC Research, and in January 2016 a second study on basic income was also announced.