Well there are a lot of good definitions around here allready. But most people forgot the many different variations of how it's written.
Why do people use this variations? Well you know this whole urban thingy. Everyone like to be different and original. That's why some use these variations. Some allso use it bacause they believe variations such lolz are funnier.
here a list of some variations:
lawl, lolz, lel, lelz, lul, lal