Github is a proprietary website hosting git repositories which are used to store all changes of a project.
Also see version-control-systems git
It is still friendly to open source people, because hosting for public repositories is free. All you need is an account, then you can pick a project, hack on it, and provide patches via pull requests. Once you get used to it its pretty simple which is why the source of this wiki is hosted that way, too.
Most people know git reasonable well which is why this wiki is using it.
For this and similar reasons its very popular among open source developers. There are alternatives such as bitbucket and gitorious etc.
The github page of this wiki currently is http://github.com/MarcWeber/vim-git-wiki which might change in the future.