The term "n00b" is commonly confused with the term "newb".
A newb is short for a newbie, someone who is new to something.
A n00b is someone who, regardless of experience, is utterly crap at a game and will not take the advice of others.
These are generally big mouthed people who suffer from Delusions of Grandeur
Upon being given advice, n00bs will either ignore it completely, or make a poor attempt at insulting the person giving them the advice.
A n00b will be the first person to point the finger at someone else if a mistake is made.
They are also the people that go around calling others n00bs for no particular reason.
90% of n00bs are unable to spell.
The word n00b is also commonly used in banter.