http://wildlyinaccurate.com/a-hackers-guide-to-git
From the post:
Complex systems like Git become much easier to understand once you figure out how they really work. The goal of this post is to shed some light on how Git works under the hood. We’re going to take a look at some of Git’s core concepts including its basic object storage, how commits work, how branches and tags work, and we’ll look at the different kinds of merging in Git including the much-feared rebase. Hopefully at the end of it all, you’ll have a solid understanding of these concepts and will be able to use some of Git’s more advanced features with confidence.
It’s worth noting at this point that this guide is not intended to be a beginner’s introduction to Git. This guide was written for people who already use Git, but would like to better understand it by taking a peek under the hood, and learn a few neat tricks along the way.