Great Britain is an island containing England, Scotland and Wales. - Edit 2
Before modification by Joel at 19/11/2012 08:08:26 PM
I found it really useful, and you might too if you're not familiar with the differences.
Ireland is the island to the northwest, part of which Britain aggressively occupied centuries ago. Collectively, they are "The United Kingdom of Great Britain and Northern Ireland." It is pretty straightforward, but I understand Scots dislike being called "British" (though the term is technically correct for both them and the Welsh.)