In this presentation from Scott Chacon, author of Pro Git and evangelist at GitHub you'll get everything you need know about Git. Below is Scott's description of the talk:
This talk introduces the Git Version Control System by looking at what Git is doing when you run the commands you need to do basic version control with it. We'll look at how to use Git to do the basics, while seeing how it differs from Subversion, what staging and committing actually looks like, how it stores it's data, how it branches and merges so nicely and how it talks to a server when pushing and fetching. Then we'll look at how to look at your history with log in interesting ways. This should help Git newbies get acquainted with the popular VCS and other Git users get a glimpse of what's happening under the hood.