A. someone new to said game("noob" can but does not correctly apply to other aspects of life outside the range of video games.)
B. someone who is bad at said game, commonly someone who has played said game for a decently long period, or more who claims to be "good, nice, sick, beastly, epic" at said game and then gets OWNED over and over again will commonly rant on calling every one who kills, beats, and or defeates him or her a "noob".
C. someone being called a "noob" by a noob because the real noob is angry as his or her noobness and retaliates by calling the non-noob a noob.
Player1(noob): Hey man how do you get up there?
Player2: Wow noob, go play runescape!
Player1(noob):*dies(constantly)* WTFF DUDE YOU NOOB
Player2: You mad son?
PLayer2(noobb): wtff you noob stop being lame