Hi Gary,
I've just tried a game myself (and lost!). It's normal chess but the bank rank pieces are placed in a random position. Castling is a bit different as well but aprta from that it's the same as normal chess. It's designed to make people rely on tactics and skill as opposed to opening theory (which goes out of the window!). I think! :)
Check out this link: http://en.wikipedia.org/wiki/Chess960
You can start a game by clicking on "Start New Game" within Online Chess (as normal) and click on "Show more advanced options". Then select Chess960 in the Game Type drop down box.
Cheers!
Paul.
PS Your rating is kept seperate from your online chess rating although it does seem to use the online chess rating when searching for players. A bit strange but you get used to it.
Its a different board, I will see if I can invite you.