Git is a popular version control system designed to handle very large projects with speed and efficiency; it is used mainly for various open source projects, most notably the Linux kernel, xorg, OLPC, Wine, Samba, swfdec and many others.
Git falls in the category of distributed source code management tools, similar to e.g. GNU Arch or Monotone (or Bitkeeper in the proprietary world). Every Git working directory is a full-fledged repository with full revision tracking capabilities, not dependent on network access or a central server.
Git tools under Debian
git - git itself
giggle - Gtk+ frontend for the git directory tracker
git-buildpackage - Suite to help with Debian packages in Git repositories
git-dpm - Another suite to help with Debian packages in Git repositories
There are many other tools, see git
Alioth/Git - Using Git on Alioth
GitSrc - .git.tar.gz format for debian source packages.
PackagingWithGit - Using git to maintain debian packages