A noob comes from the word Newbie. It is a general name used for those who are new to World of Warcraft. Alas, over time it has been twisted into an insult, and many Non-RPers will go around and call players a Noob if they refuse to give them money, duel, or if they see players roleplaying.
People using such language as 'noob' openly are considered a general annoyance, so many players ignore them. If they are particularly abusive, Blizzard can help players with the issue.