1. Someone who is in-experienced at something
2. A word overused by geeks with delusions of superiority who think they are cool because they have no life and play computer games 24/7. It is often, for no apparent reason, spelled with zeros instead of O's, another geek thing, e.g. n00b instead of noob.
"Ha ha ha, that kid has a life and isn't a grand master at this game like me, what a n00b!"