What really helped me understand Git was this 2-part tutorial that uses Gitk to show how your commits and merges work.

Part 1 - http://lostechies.com/joshuaflanagan/2010/09/03/use-gitk-to-...

Part 2 - http://lostechies.com/joshuaflanagan/2010/09/03/use-gitk-to-...

I'm surprised how few tutorials mention Gitk.