In my career I've been exposed to a rather limited set of version control systems. I've used CVS, Subversion, SourceSafe and Accurev. Each of those systems are based on a central repository with many clients. The concept is a simple client-server model that is very easy to understand.
Git is a "Distributed Version Control" system. This means that everyone effectively has their own repository of the code that they can fork and merge as much as they'd like locally. DVCS systems like git work on a peer-to-peer model. This is a very powerful feature in that if I have some code that you would like to integrate with I can exchange it with you, and you alone. In a VCS system like Subversion or CVS, I would have to commit my changes to the central repository for you and everyone else to get. Sure, I could branch and put my changes there, but merging that back in is a huge pain in Subversion and CVS.
InfoQ just posted a lengthy article discussing Distributed Version Control Systems such as git, mercurial, and bazaar.