The key difference is that it is decentralized. Imagine you are a developer on the road, you develop on your laptop and you want to have source control so that you can go back 3 hours.
SVN has the advantage that it’s MUCH simpler to learn: There is your repository, all changes to towards it, if you know how to create, commit and checkout and you’re ready to go and can pickup stuff like branching, update etc. later on.
Git has the advantage that it’s MUCH better suited if some developers are not always connected to the master repository. Also, it’s much faster than SVN. And from what I hear, branching and merging support is a lot better (which is to be expected, as these are the core reasons it was written).
Git has good performance because operation like diff is done locally.
Subversion is fine.. until you start merging.. or doing anything complicated.. or doing anything Subversion thinks is complicated (like doing queries to find out which branches messed with a particular file, where a change actually comes from, detecting copy&pastes, etc)…
A good article points to the linus video on google talk in 2007