Random - but ill bite. This is actually right up my alley, I work in the industry.
GIA is widely regarded as the worlds authority on diamond grading. They were the creators of the 4C's which is now accepted as the standard by which nearly all diamonds are graded. They are an education and research institution that teaches people to become gemologists. They have no retail entity and do not sell any of the diamonds that they grade.
A diamond grading report serves as your guide to the diamond. The problem with diamonds is that with the naked eye or even with a loupe, unless your a trained professional it is very hard to distinguish between a diamond that costs several hundred dollars and a diamond that costs several thousand dollars. With a grading report from an impartial independent lab you can be sure of exactly what your getting.
Also, most insurance companies require a grading report if you ever want to get your diamond insured.
gia4cs.gia.edu is the site to check out if you want more information on the 4C's of diamonds and what they mean.