In the Neopets realm: Someone tht is purposly annoying person that is usually illiterate, spamming, begging, etc.
In the multiplayer online gaming world: Someone that is purposly anoying, usually illiterate, spamming, teamkilling, camping, rocket spammer, etc.
(NOTE: Unlike n00b, which derives from newb, a y00b is a player who has experience in whatever field of gaming they are in, and yet still suck and/ are annoying.)
Hey, my teammate in Halo shot me in the back and ran away! What a y00b.