IKR: I Know Right?
ROFLMAO: Rolling On Floor Laughing My Ass Off.
Z is also added on words just for the internet. Usually verbs. Ex: i lolzed at tat pic! OMFG (Oh My F*cking Gosh)
If you need any more, I have plenty (I sound proud of that!XD)
Ah yes: XD Means nothing. You probably know smileys though...
Edit: Y: Just shortened Why?
Hey, they forgot one: TSS. It stands for "Text-Speak Sucks". Also, as ROFL is closely related to ROFLMAO, so is TSS closely related to TSSMER, "Text-Speak Sucks Major Elephant Rear".
Yeah...I think the only reason you'd ever need to learn these is to translate them, personally, although I think I've at least caught myself saying TMI (Too Much Information) a couple of times. (Only when I'm around my sister, who is a linguistic nightmare, but I haven't said it in quite a while.) Um, yeah...JK is for "Just Kidding", although I haven't heard anything here say that (thank goodness), and NSFW stands for "Not Suitable For Work", which is simply a euphemism for "So perverted/inappropriate/disturbing that it would be rated NC-17".