A "noob" is a person who is new or inexperienced in a subject, usually an online game. Noobs are usually annoying and excessively stupid. Do not confuse noobs with newbs, who are inexperienced players that tend to be more mature and strive to become better.
Noob: can i hav muneys pplsplsplsplzz!!11
Newb: Would you mind sparing me some money?
Noob: pl0x sir!! money plszzz!!!111111111
Newb: Okay. :(