This article is from the The Simpsons FAQ, by Ondre Lombard wakkojr@nospam.wcat.com and Andrew Gill superluser@mail.rit.edu with numerous contributions by others.
(almost verbatim from LISA):
Acronym Translation
a.t.s - The Usenet newsgroup alt.tv.simpsons
MG - Matt Groening
OFF - Our Favorite Family (The Simpsons, of course)
SLH - Santa's Little Helper (the Simpsons' family dog)
DYN - Did You Notice, Didja Notice
I&S - Itchy and Scratchy.
FFF, FF - Freeze-Frame Fun
SNPP - Springfield Nuclear Power Plant
IMHO, IMO - In My (Humble) Opinion
BTW - By The Way
WRT - With Regard To, With Respect To
ROFL - Rolling On the Floor Laughing
IYKWIM - If You Know What I Mean
AFAIK - As Far As I Know
WTF - What The [Heck]
IIRC - If I Recall Correctly
RTFF - Read The [Darn] FAQ!
Other Terms:
Production Code/P-code - The four character episode number given to
each episode (e.g.: 7G08, 7F24, 2F05)
Couch Gag - Part of the title sequence when the family
runs into the house and something unusual
happens.
Ref - Reference, where an event or scene in the show
parodies another show, movie, or event.
Blackboard Quote - What Bart writes on the chalkboard at the
beginning of the title sequence.
Episode Capsule - A compilation file for each episode.
 
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