Lol -> lul; lols -> luls; lolz -> lulz.
Lulz is the one good reason to do anything, from trolling to rape. After every action taken, you must make the epilogic dubious disclaimer: "I did it for the lulz".
This has been pioneered by encyclopedia dramatica, famous for posting a fake craigslist add and then listing the personal info of those who responded.
2. An idiotic excuse to do ANYTHING offensive or disturbing (and as such, funny) used mostly by Chantards (4chan, IIchan, etc.) Gaiafags, or any usual member of Encyclopedia Dramatica.
3. Something funny that is caused at the expensive of others.
Troll: i didz it for da lulz
/b/tard: dude whyd u cum on ur sisters face and then post pics?
/b/tard(2): i did it for da lulz