This can be split into a couple of different terms.
OMG = Oh My God!
ROFLMFAO = Roll On Floor Laughing My Fucking Ass Off
LMAO = Laughing My Ass Off
LMFAO = Laughing My Fucking Ass Off
Basically in a way its a dragged out way of exclaiming that something is the funniest thing you've ever seen.
1. OMFGROFLMFAO! Thats too funny!!
2. Random user 1(11:03:55 PM): dude, my fish just died
Random user 2(11:04:13 PM): OMFGROFLMFAO!!!
Random user 1(11:05:27 PM): not funny :(