Lulz is a variation of "LOL" or "Laugh Out Loud" or just "Laughs".
Lulz is often used to denote laughter at someone who is the victim of a prank, or a reason for performing an action. On the internet "lulz means the joy of disrupting another's emotional equilibrium."
Lulz can be used as a noun — e.g. "do it for the lulz.", shortened into "ftlulz" (to distinguish it from "ftl" - "for the loss").