Evolving from the word New-byers, meaning new passing byers in town, this changed to newbie, the shortened form of this is newb.
A newb is a word used over the internet many times, usually meaning new to a subject/topic, usually a game.
A noob was evolved through the uneducated side of games played by usually children, runescape
for example, where children didn't know the exact spelling of newb and changed it to noob.
Newb and noob shouldn't be confused however, newb is a new person to a game but wants to become better and be like the higher levels in a game. A noob is a new player who tends to be aggrovating and will usually spam, beg, or find any other way to annoy other players.
As not many people like those kind (noobs), the term has become something over the internet to become offensive.