you again, huh? lol
well it's in the online live faq (read the board/rules when you join the xbl dashboard)
It goes quite in depth..
Basically the idea is you take rank points away from your opponent when you win and vice versa, and the stronger your opponent, the more percentage of his points you get. A top 100 player, needs to play high ranked players to stay on top etc..
edit: oh and as far as stats etc.. they are just kept for your tracking purposes, they don't affect rank points according to the vet gamers I spoke to..