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.
The Urban Dictionary Mug
One side has the word, one side has the definition. Microwave and dishwasher safe. Lotsa space for your liquids.
Buy the mug