Git is a distributed source version control tool.
Martin Fowler mentions it in his blog post, where he describes his thoughts on the topic. There's a lot of good links in that post. Here are a few more useful ones...
Git Community Book - the heart of documentation,
and, of course, TortoiseGit download page.
If you’d like to use git in a fashion similar to CVS (or SVN), read gitcvs-migration.
Also, here is the link to Mercurial home.