These abbreviations aren't used just here. They are standard for most message boards on the Net. They save on typing.

Off the top of my head, these are the most common.
DH - darling (darn) husband
DW - darling (darn) wife
DS - darling son
DD - darling daughter
MIL - mother in-law
FIL- father in-law
BIL - brother in-law
SIL - sister in-law
FWIW - for what it's worth
BTDT - been there done that
BTW - by the way
OT - off topic
OP - original poster
GMTA - great minds think alike
LOL - laugh out loud
ROFL - rolling on the floor laughing
LMBO - laughing my butt off
JMHO- just my humble/honest opinion
NAK -nursing at keyboard
There are many more that specific interest groups tailor to their needs. For example, I'm a Child Passenger Safety Technician and belong to a few car seat boards. BPB - belt positioning booster, OHS - over head shield, HWH - high weight harness, ...