Some of us aren't as good at chat-speak as others, and so, it's easiest to have a guide. If you find another that isn't on here, please post it!
AKA - 'Also known as'
ASAP - 'As soon as possible'
B4 - 'Before'
BBL - 'Be back later'
BIC - 'Back in Character' as opposed to 'Out of Character'. Used to confirm that one is role-playing, when before they were not.
BRB - 'Be right back'
BTW - 'By the way'
Bump - An acronym for 'bring up my post'. When a topic does not receive any replies for a while, the creator may bump it to make it easier to find and possibly more popular.
C&P - 'Copy and paste', used to show the actions of copying and pasting.
FAQ - 'Frequently Asked Questions', a link to the official forum FAQ can be found here.
FTW - 'For The Win', often used to show support over one thing or to show that one thing is better than another.
GTG/G2G - 'Got to go'
IC - 'In Character' as opposed to 'Out of Character', used to show that one is back in character, or role-playing, when he or she has recently been out of character.
Imma - 'I am'
JK - 'Just Kidding'
K - short for 'okay' or 'OK'
LOL - 'Laugh out loud', used to show laughter
NM - 'Nothing much', used as a reply to "What's up?" or anything along those lines.
NP - 'No problem'
OMG - Most commonly used as 'Oh my God' but also can be used as 'Oh my Gosh'
OOC - 'Out of Character' as opposed to 'In Character', used to show that one is not role-playing in a situation where most members in that section of the forum are role-playing.
Pinned - A Pinned topic is an important one that is not quite an Announcement.
PM - A private message using the messaging feature.
ROFL - 'Rolling on the floor laughing', used to express laughter; the person is unable to stand for mirth
ROFLAO - 'Rolling on the floor laughing my ass off', used to express laughter; the person is unable to stand for mirth
RP - Role-play
TY - 'Thank you'
WB - 'Welcome back', used when someone has left for a short amount of time or is on vacation.
YW - 'You're welcome'