I recently found this presentation by Scott Chacon and I personally find it very intuitive in helping to understand how GIT works under the hood. He explains in a very neat way how the concept of pointers to snapshots works and imho it’s an hour of worth while watching. If you’re new to GIT it will also provide an explanation of 12 most used commands (this is a Scott’s direct quote: ‘…commands that you’ll be using 95.6% of a time, this is very scientifically based…‘ 🙂 ).
Also Scott wrote and published a book called ‘Pro Git’ which is free for download at git-scm.com/book.