GIT is a distributed revision control system that allows to working with the versioned files locally and remotely. It is something different then SVN, but its main functionality is quite similar. GIT was designed to increase performance of typical, for each revision control system, operations like: merging, pushing, pulling branches.
There are a lot of books, e-books and articles about GIT, but for the people who never heard about it Pro GIT seems to be a very good choice. After reading Pro GIT we will be ready to start using this system.