What are the core concepts of git, github, fork & branch. How does git compare to SVN?

Question

I\'ve been using Github for sometime now but I\'m getting a bit confused about some key concepts behind Git.

My background with VCS started with Source Safe and then

Answer 1

There is a nice talk about the internals of Git by Scott Chacon.

A slideshow with voiceover: http://blip.tv/scott-chacon/git-talk-4113729
The slides again: http://www.slideshare.net/chacon/getting-git
A live recording of the talk: http://vimeo.com/1099027