What is the difference - UK, GB and Britian?

This guy speaks pretty fast, but his description is very clear cut and easy to follow.  I love his blog and will be posting more links to it in the future.

http://blog.cgpgrey.com/
http://blog.cgpgrey.com/the-difference-between-the-united-kingdom-great-britain-england-and-a-whole-lot-more/