Git is an open source version control system designed to handle very large projects with speed and efficiency, but just as well suited for small personal repositories; it is especially popular in the open source community, serving as a development platform for projects like the Linux Kernel, WINE or X.org.
Git falls in the category of distributed source code management tools, similar to e.g. Mercurial or Bazaar. Every Git working directory is a full-fledged repository with complete history and full revision tracking capabilities, not dependent on network access or a central server. Still, Git stays extremely fast and space efficient.