Joel Spolsky has put together an extensive Mercurial tutorial with lots of examples and a humorous approach. "Finally, if you haven’t collapsed yet from exhaustion, before I finish this tutorial, I just want to show you one more tiny thing: you can use the hg update command to go backwards or forwards in time to any revision you want. Well, you can’t really go into the future per se, although that would be super-cool. If you only had four revisions you would just hg update -r 103994 and get some really cool anti-gravity sci-fi futuristic version of your source code. But of course, that is not possible."
Read more at LWN