Drupal now offers git to access its source code and 3rd party modules, themes, etc.
The GIT system is interesting but it has a huge difference between the CVS and SVN scheme: it requires an action, such as commit, and a push. The push is what synchronize your copy with the repository. Without the push, all the actions you've been taking will have no effect (they only are on your hard drive.)
As you will see in each project, there is a Version Control tab at the top. Clicking on it gives you most of the instructions you need to know to work with Drupal git. I will not repeat them here. The only two things I often do is commit + push and tagging (to create new official versions.)