I stole this from Nyth:
MWS,New 3 Letter,DESCRIPTION
A,LEA,Limited Edition Alpha
B,LEB,Limited Edition Beta
U,2ED,Unlimited
R,3ED,Revised Edition
4E,4ED,Fourth Edition
5E,5ED,Fifth Edition
6E,6ED,Classic (Sixth Edition)
7E,7ED,Seventh Edition
8E,8ED,Core Set - Eighth Edition
9E,9ED,Core Set - Ninth Edition
10E,10E,Core Set - Tenth Edition
AN,ARN,Arabian Nights
AQ,ATQ,Antiquities
LG,LEG,Legends
DK,DRK,The Dark
FE,FEM,Fallen Empires
HL,HML,Homelands
IA,ICE,Ice Age
AL,ALL,Alliances
CS,CSP,Coldsnap
MI,MIR,Mirage
VI,VIS,
VisionsWL,WTH,
WeatherlightTE,TMP,Tempest
SH,STH,Stronghold
EX,EXO,Exodus
US,USG,
Urza's Saga
UL,ULG,
Urza's Legacy
UD,UDS,
Urza's Destiny
MM,MMQ,Mercadian Masques
NE,NMS,Nemesis
PY,PCY,
ProphecyIN,INV,Invasion
PS,PLS,Planeshift
AP,APC,
ApocalypseOD,ODY,Odyssey
TO,TOR,
TormentJU,JUD,Judgment
ON,ONS,
OnslaughtLE,LGN,Legions
SC,SCG,Scourge
MR,MRD,Mirrodin
DS,DST,Darksteel
FD,5DN,Fifth Dawn
CHK,CHK,Champions of Kamigawa
BOK,BOK,Betrayers of Kamigawa
SOK,SOK,Saviors of Kamigawa
RAV,RAV,Ravnica: City of Guilds
GP,GPT,Guildpact
DIS,DIS,Dissension
TSP,TSP,
Time SpiralTSB,TSB,
Time Spiral Timeshifted
PLC,PLC,
Planar ChaosFUT,FUT,
Future SightLRW,LRW,Lorwyn
MOR,MOR,
MorningtideSHM,SHM,Shadowmoor
EVE,EVE,Eventide
ALA,ALA,Shards of Alara
CFX,CFX,
ConfluxCH,CHR,Chronicles
AT,ATH,Anthologies
BR,BRB,Battle Royale
BD,BTD,Beatdown
DM,DKM,Deckmasters
EVG,EVG,Duel Decks: Elves vs. Goblins
DRB,FVD,From the Vault: Dragons
JVC,JVC,Duel Decks: Jace vs. Chandra
PT,POR,Portal
P2,P02,Portal Second Age
P3,PTK,Portal Three Kingdoms
ST,S99,Starter 1999
S2K,S00,Starter 2000
AS,AST,Astral
MED,MED,Masters Edition
ME2,ME2,Masters Edition II
UG,UGL,Unglued
UNH,UNH,Unhinged
TK,TKN,Tokens
CST,CST,Coldsnap Theme Decks
BCE,BCE,Collector's Edition
4EA,4EA,Fourth Edition (Alternate)
I2P,I2P,Introductory Two Player: Kazz & Zakk
MV,MUV,Multiverse
DC,SDC,Dreamcast (Sega)
BT8,BT8,Box Toppers 8E
BTP,BTP,Box Toppers Promo
FI,FIL,Filler Cards
OV,OV6,Oversized 6x9
12,OV9,Oversized 9x12
WP,WPM,Misprints-Wrong Pics
TP,TPR,Test Prints
VG,VGD,Vanguard
VGM,VGM,Vanguard (MTGO)
ARE,ARE,Promo –
Arena League
CHP,CHP,Promo – Champs
FNM,FNM,Promo – Friday Night Magic
GTW,GTW,Promo – Gateway
JGC,JGC,Promo – Judge Gift Cards
JSS,JSS,"Promo – Junior Super Series, Junior Series, Junior APAC Series, Magic Scholarship Series"
REW,REW,Promo – Magic Player Rewards
PRE,PRE,Promo – Prerelease
REL,REL,Promo – Release
PR,PRO,Other Promos
ARC,ARC,Armada Comics Promos
DLM,DLM,DCI Legend Membership
HHL,HHL,Happy Holidays Promos
HPB,HPB,Harper Prism Books Promos
SUM,SUM,Summer of Magic Promos
THG,THG,
Two-Headed GiantSM,SMM,Summer Magic
APL,APL,Asia Pacific Land Program
EUL,EUL,European Land Program
GUR,GUR,Guru Lands
MPS,MPS,Magic Premiere Shop Lands