Acronyms are often substituted for commonly used expressions. Here are a few of the more common ones that you might see in Be Communities:  

AFAIK As Far As I Know
AKA Also Known As
ASAP As Soon As Possible
BTW By the Way
ETA Estimated Time of Arrival
FAQ Frequently Asked Question
FWIW For What It's Worth
FYI For Your Information
HTH Hope This Helps
IMHO In My Humble Opinion
IOW In Other Words
NDA Non-Disclosure Agreement
NRN No Reply Necessary
OTOH On the Other Hand
PMFJI Pardon Me for Jumping In
TIA Thanks in Advance
WAD Works as Designed
WRT With Respect To

You can find definitions of other acronyms at Web sites like http://www.acronymfinder.com/ or http://www.ucc.ie/info/net/acronyms/