Derrives from "Dudes", wich bassicaly means "Persons".
Often used in games, forums and chats.
It is more like an internet slang, started back in 2006, when a Counter-Strike player asked "Where are you", and the other replied "Im in Ur base, killin Ur doodz".
t3hL33tz0r3r: kan U c those doodz??
l337h4x0r3r: yeh i c dem