What is Git?
Git is a stupid content tracker. That is probably the best description of it - don't think of it in a 'like (insert favorite SCM system), but...' context, but more like a really interesting file system.
Git tracks content - files and directories. It is at its heart a collection of simple tools that implement a tree history storage and directory content management system. It is simply used as an SCM, not really designed as one.
NOTE
"In many ways you can just see git as a filesystem --- it's content-addressable, and it has a notion of versioning, but I really really designed it coming at the problem from the viewpoint of a filesystem person (hey, kernels is what I do), and I actually have absolutely zero interest in creating a traditional SCM system." - Linus
When most SCMs store a new version of a project, they store the code delta or diff. When Git stores a new version of a project, it stores a new tree - a bunch of blobs of content and a collection of pointers that can be expanded back out into a full directory of files and subdirectories. If you want a diff between two versions, it doesn't add up all the deltas, it simply looks at the two trees and runs a new diff on them.
This is what fundamentally allows the system to be easily distributed - it doesn't have issues figuring out how to apply a complex series of deltas, it simply transfers all the directories and content that one user has and another does not have but is requesting. It is efficient about it - it only stores identical files and directories once and it can compress and transfer its content using delta-compressed packfiles - but in concept, it is a very simple beast. Git is at its heart very stupid-simple.