Pwned is a word that was most likely created by a noob on a video game that typed "pwned" instead of "owned."
It has become used by non-noobs, perhaps to make fun of the person that made it, perhaps because it is so lacking in proofreading, it is hilarious.
lykomg u got pwned dood - noob usage
Haha, that teacher totally pwned you, man. - Everyday usage