Like many other "0r" words (like bitchz0r, or hax0r, or sux0rz), this has an unknown origin. But it can be traced back to "lolsaur".
If you are a true lamer, use it. If not, AVOID IT AT ANY COST, or people will start to think you're a moron.
Person1: something REALLY funny
Person2: OMG lolz0rrrr!!!!111
Person1: yaaaaaa bitchz0r die cuz i am teh hax0rz
Person2: kekeke! lolz0r!!!11
Person2: lolz0r!!!111 kekeke omg lolz0r!!!
lolz0rs originates from the original phrase used by 9 year olds, lol, which means "Laugh out Loud" but this particular variant is used by geekdorks, startrek/scifi fans and the like..
"lolz0rs I am teh Bork!!!11one"