The basic 'lol' is what is generally used.
Lol=Laughing out loud
'Rofl' is a step up from 'lol'.
Rofl=Rolling on [the] floor laughing
'Roflmao' is the 3rd step up. For the tender-worded type, a can be substituted for b.
Roflmao=Rolling on [the] floor laughing my ass[butt] off
'Roflmaowtime' is the 4th step up, and rarely used. Once again, a can be substituted for b.
Roflmaowtime=Rolling on [the] floor laughing my ass[butt] off with tears in my eyes
'Lmso' is for the younger branch of web chatter.^^
Lmso=Laughing my socks off
Farewells-
'G2g' is heard everywhere when you must get off your computer. :( Some people use a t instead of a 2, but they mean the same thing.
G2g/Gtg=Got to go
'Ttyl' is normally the reply you get when you say 'g2g'
Ttyl=Talk to you later
'Ttfn' is very similar to 'ttyl', but not its not the same! Don't forget!
Ttfn=Ta ta for now
'Brb' is somewhat of a farewell, but you will be back shortly.
Brb=Be right back
'Bbl' is the next step up from 'Brb'. When you're gone for a longer amount of time.
Bbl=Be back later
'Cya' is quite simple, but we've been told some people don't know what it is.
Cya=See ya(and for the F grade average, ya means you)
Affections-
'Lyl' is a term we've picked up recently.
Lyl=Love you lots
'88' is a term Okibi just learned herself! Quite useful, too.
88=Love and Kisses
'HH' is one we picked up from a show..never seen it used though.
HH=Holding hands
'<3' is a sign becoming increasingly popular among chatters.
<3=A heart, which also means love.
Misc-
Ty=Thank you
Yw=Your welcome
Ic=I see
Wb=Welcome back
J/k=Just kidding
J/w=Just wondering
N/m=Never mind
N/m=Nothing much
A/s/l=Age, sex, and location(some can be taken out, such as, if you know the age, say s/l. Or their gender, a/l. Or location, would be a/s)
Updates
Sos=Sibling over shoulder
Sd and St=Sweet dreams and Sleep tight
OOC=Out of character
OT=Off topic
If you would like to add some Slang, email 'Kibi.