The ones you listed there are all basically the same thing. They are just different representations of smileys. Like the p being a tongue stickin out y'know. The weird symbols like the upside-down question mark and the r symbol don't really mean anything, that is just most likely some kids who know the key combination to make weird symbols and wanna look cool. As for some common ones.....
BRB = be right back
BRT = be right there
OMG = oh my god
OMW = on my way
AFK =away from keyboard
AFAIK = as far as I know
IMO = in my opinion
LOL = laughing out lol
ROFL = rollin on floor laughing
LMAO = laughing my ass off
W T F = What the ****
That's all I can think of right off the top of my head. If you see any others you're unsure about just ask.
BRB = be right back
BRT = be right there
OMG = oh my god
OMW = on my way
AFK =away from keyboard
AFAIK = as far as I know
IMO = in my opinion
LOL = laughing out lol
ROFL = rollin on floor laughing
LMAO = laughing my ass off
W T F = What the ****
That's all I can think of right off the top of my head. If you see any others you're unsure about just ask.