Don't worry, I had the same confusions as you when I first joined here! I'll do my best to help, so, here goes:
GAC is a Guest Assistance Card. These are designed for people with invisible disabilities (covering conditions as wide ranging as visual impairment and autism), but are not required for people using a wheelchair (unless they have other, unseen problems, besides mobility issues). The FAQ (
http://www.disboards.com/showthread.php?t=595713 ) has loads of information on it, but if you have any more questions, ask away.
And
ECV is an Electric Convenience Vehicle (or words to that effect) and is another name for a mobility
scooter. They can be rented on or off site, but only for people over the age of 18.
I would guess SN is special needs, but I don't think I've really seen that one floating about.
Also a couple more generic Disney abbreviation that might help you:
MK: Magic Kingdom
AK: Animal Kingdom
DDP:
Disney Dining Plan
POFQ: Port Orleans French Quarter
PORS: Port Orleans River Side
AS: All Stars
CBR: Caribbean Beach Resort
CSR: Coronado Springs Resort
FW (sometimes FtW): Fort Wilderness
WL: Wilderness Lodge
etc...
I'm sure I'll think of a few more soon, and add them as appropriate.
Hope this helps

!