Y Combinator is the startup accelerator founded by Paul Graham and Jessica Livingston in 2005. It began by funding small groups of startups with seed money, guidance, and a shared program.
The accelerator became influential in Silicon Valley by standardizing a short, intensive process that ends with Demo Day, where founders present their companies to investors. Its alumni include Airbnb, Dropbox, Coinbase, DoorDash, and Reddit.
Y Combinator is often confused with venture-capital firms such as Sequoia Capital, but an accelerator typically works with cohorts of very early-stage companies. The organization also popularized a model in which founders receive a small investment in exchange for equity and access to a strong founder network.