"n00b" is a common internet slang word for someone who is irritating to other users and never seems to relent in their behavior. The term derives from "newb", which is short for "newbie". People often confuse the two terms, unintentionally insulting someone who just hasn't been at the boards long enough to know any better, which is the general definition of newb.
n00bs are often compared to trolls, because they tend to exhibit the same kind of annoying behavior. Typical n00b activities include asking questions they know the answers to, generally ignoring the ToS, being disrespectful to other users, having extremely poor grammar and spelling, and just annoying people with their immature behavior.
Calling someone a n00b is often considered as flaming. Other similar terms include nub (which is generally considered more playful, but moderatable), nublet, or n00blet.
n00b, however, may also be wrongly used by elitists who are usually insecure about themselves, or over-eager new users to a board who want to gain respect from the more well-established users of the board.
The word "n00b" can also be used to describe other people in an online game, and has a similar meaning.