Okay...
Oeridians = Romans
Flan = Celts (or Native Americans, there's a blend)
Baklunish = Arabians
Suloise = Germanics
Olman = Aztecs
Rhennee = Gypsies
Now, that's a very basic breakdown, and plenty of people disagree. Culture (and nation) is far more important in Greyhawk than skin colour, but it does apply.
Each race has its own pantheon, but over the years, they have become somewhat mixed. (Something that should have been in the LGG is a list of deities by pantheon).
The Baklunish and Suloise participated in a great war 1,000 years ago that ended in the ruin of their civilisations, the Oeridians then took over and founded the Great Kingdom of the Aerdy - this is now in decline, much like ancient Rome.
The Suloise are fair-skinned; some of them are viking-like (the Snow, Ice and Frost Barbarians), which is why I equate them to the Germanics/Aryans.
The Flan are the original inhabitants of the Flanaess, now largely displaced by the invaders of the other three races. They had a druidical culture, which is why I normally equate them with the Celts; some versions of them act a bit like Native Americans (the Nomads, tiger & wolf).
Rhennee are wanderers originally from another world (Rhop?). They travel on barges, and have tight family clan groups - very gypsy-like.
The Olman aren't really seen very much of.
There may be a couple of other minor races, but that's the gist of it. There are no game differences between the races.
Cheers!