git is a source code management (scm) system, just like cvs or svn (subversion). git is the source code management system developed (initially) by Linus Torvalds for managing the Linux Kernel Source code. See
http://git.or.cz/.
A source code management system adds a history to your source code. Each change is "commited" and then such a commit has a date, and author, a diff to the previous state of the source tree, a commit message explaining the change, and so on. The source code history is then the chronologically ordered series of commits. This allows you to easily roll back changes.
To get an idea, you can browse the source code and the history on the web here:
http://git.amahi.org/?p=amahi.git
But unlike the more traditional SCM systems cvs and svn, git follows a decentralized approach. In this respect it is more like bazaar (bzr) and mercurial (hg). Decentralized means, that there is not one central server to which you need to be connected in order to commit your changes, but you clone the repository and work locally instead. The mainainer of the official branch also commits locally and then pushes his changes to the upstream repository. A contributor can publish his changes in the ways detailed in a previous post (email or a private repository), and the maintainer of the upstream repository can pick his changes and push them upstream. The advantage is that in contrast to cvs and svn, authorship of the commits is kept when picking changes, i.e. you do neo need to have commit access to the central repository in order to appear as an author in the central repository's history...
@moredruid: is this sort of what you wanted to hear?