An "l33t" person is an american teenager that gets picked on at school.
They use spare time to "own" people (similar to the way they get "owned") in words on the internet.
This usually takes place on forums (message boards), internet multiplayer games and IM (mostly IRC).
They usually **sound** tough, but you should re-read what they typed, as its usually calling someone a pussy (or something similar) ON A GAME.
Their weakness usually shows when you respond with something todo with the real world, eg: school, money, looks, girlfriend etc. Expect a "shut up" or no reponse (or leaving the online game).
l33t : haha, pwn3d!
reply: not as much as you, sunday today...school tomorrow!