I'll recommend reading this nice and deep explanation after using git for a while.
This is not an introduction, but an intermediate guide about how git works, and a great start to understanding what's happening inside the .git folder.
This is a very nice small document. Given its size, many things are viewed only superficially, but the important notions are there. It's not a first introduction to git IMO, as it's more concerned with understanding what's going on behind the scenes rather than everyday use. One complaint is that sometimes there are references to things not yet presented (at the end for example, when talking about stash), and sometimes definitions are not clear enough. In those cases, Google is your friend. After reading this, Pro Git can be the next step.