Everybody else here is entirely incorrect, and it's unbelievable to what a degree.
"LOL" is the signular form, and "LOLZ" is simply the plural.
Meaning "lol's" or more than one "lol".
Adding a lowercase "z" at the end (Proper form, LOLz, or LoLz to switch between capitals and lowercase as is commonly done by a number of prepubescent horny teens, namely girls) does not necessarily add another word to the acronym. Just like if I were to write to you about "STDs", I would not be talking about Sexually Transmitted Disease Succotash, but rather multiple Sexually Transmitted Diseases.
Here in teh intranets, teh misconsepshins are teh commonplace, LoLz.