A n00b is one who is unskillful or new to anything at all. Originated from the word NEWBIE (someone who is new to something), which was shortened to newb, then phonetically spelled with two o's, then by a random twist of fate, had zeros replace the o's. N00b is also primarily found in online based videogames such as counterstrike and halo. Many times after pwning a n00b you teabag him, in order for him to realize his n00byness.
A n00b can also be used as a basic insult, that is applicable to anyone, anytime, anywhere; and is often followed up with a tagged phrase.
(to an unskilled person or as a general insult)
"You are a n00b!"
"Wow you are a n00b... i've seen a walrus with a peg-leg that is less awkward!"