I'm not sure if there is a 100% set defenition or if its generally more used simpley as an insult.
If I were ever to use it (ha!) What I would mean is:
Newb: short for Newbie, someone who is new to a location.
n00b/noob: Somebody who despite the time they've spent on a site still acts like they are new.
newb is not an insult, n00b/noob is.
That's the understanding I've come to have from my various message borads and forums I've been to.