GD - Lame question, I know...

Hi team... I've seen a few posts talking about this GD thing, and thought I might try to do some reading to find out more about it... however searching the PHP docs didn't turn much up, and doing a Google search on "GD" sent back the kind of results you'd expect on a search criteria that vague... Can someone tell me a good spot to start from to learn about what GD is/does/etc. ? TIA, Jason =)