From Their Site

What is it?

Git is a fast, efficient, distributed version control system ideal for the collaborative development of software.

GitHub is the easiest (and prettiest) way to participate in that collaboration: fork projects, send pull requests, monitor development, all with ease.

How does it work?

Get up and running in seconds by forking a project, pushing an existing repository, or starting fresh. No approval is necessary.

GitHub was written for public, open source projects and private, proprietary codes — if you use Git, GitHub is for you.