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Git is a distributed version control system. It is designed to be fast, simple and powerful. It was created by Linus Torvalds, after the authors of BitKeeper fucked them over, and is used to track the source code of the Linux kernel as well as its own source code. For a colourful and interesting talk about the creation of Git by Torvalds himself, check out this video.
Git is one of the most popular development tools in use today. Lots of people contribute hack on code together through platforms such as GitHub and Bitbucket every day.