Londongirl -
As far as what the greek system is, well, from what I know you could kind of compare it to your dining/dinner societies over there. I don't know a whole lot about them, but I would say, take that idea, subtract the emphasis on dining and replace it with charity work, and you have fraternities and sororities!
For more information (and perhaps a better explanation) you might want to check out:
http://www.npcwomen.org
and I'm sure someone else could give you the website for the National Pan-Hellenic Conference and the IFC