git is a version control protocol, that is a way to store changes and versions of files so people can keep track of it, branch off and revert if things go south. It's used by programmers on collaboration projects, or those wanting to open source it for others to work on it or make use of it for free. It's not always used for collaboration, and not always for programming (the GG repo is a good example). Github, Gitlab, Bitbucket, Gitorious, Gitgud (uses gitlab software) and so on are sites that provide repository hosting, as well as additional features not part of git, like issue tracking (reporting bugs, asking devs, etc...) and a web frontend (install instructions, readme, and all that).