There's a great deal of information about 40k all over the web.  Many of it uses short hand to communicate a message.  I'm compiling a lexicon to help me decipher what it means.  Warseer uses a great system of links to know what it means.  I endeavour to copy their method in the future.  In the mean time I hope you learn as much as I do if not more.

Tasha, Suijin, Anons', Spyrle, Vargna and Hudson have provided entries.  If you'd like to provide some make a comment.  Will update this post and put credit where its deserved.

slainte mhath

++: Invulernable Save
d6: Roll 1 six sided dice
2d6: Roll 2 six sided dice
40K6: Warhammer 40,000 6th edition.  Read same for other editions.  40K5 = fifth edition.
AA: Anti-Aircraft.  Weapons that have Skyfire rule.
AB: Attack Bike, Space Marines.
AC (AssC): Assault Cannon, Space Marines.
ADL; Aegis Defense Line. 40k6 pg 114.
AFP: Airbursting Frag Projector, Tau
Alpha Strike: Tremendous amount shooting meant to cripple, if not greatly hinder, the opponent in the first round of shooting.
AoBR: Assault on Black Reach.  5th ed starter box.
AP: Armour Penetration.  If weapons AP is less than Sv target cannot roll Save.
ASM: Assault Space Marines
AtA: Air to Air
AtG: Air to Ground
ATSKNF: and They Shall Know No Fear, Space Marine.
B2B: base-to-base
BAO: Bay Area Open.  A tournament ran by Frontline Gaming in San Francisco.
BatRep: Battle Report.  Details of a game.
BFS: Battle for Salvation.  Grand Tournament held in NY.
BGB: Basic Game Book.
BiD or BID : Bring It Down, Imperial Guard Order, if successful, unit's shots against vehicles or monstrous creatures become twin-linked.  credited to Hudson
BLOS terrain: Blocking Line of Sight terrain
BP: Bolt Pistol
BotBG: Blessing of the Blood God, Khorne Gift
BRB: Basic Rulebook
BT: Black Templars, Space Marine Chapter.
BTB: Base-to-base
Bubblewrap: Unit or units used to protect other units. q.v. 12.5.9 BatRep Necron vs Chaos Daemons
BYB: Big Yellow Book.  6th ed corebook.
BWG: Beltway Gamers.  A gaming community in Washington. 
CC: Close Combat, also Chaos Cultist
CCB: Catacomb Command Barge, Necrons.
CCW: Close Combat Weapon
CD: Chaos Daemons
CF: Chainfist
Chimera ML/HF: Chimera with Multi-laser & Heavy Flamer, Imperial Guard.
Chronotek, Crytpek with Chronometron, Necrons.
CML: Cyclone Missile Launcher, Space Marines.
CoD: Cities of Death.  Games Workshop buildings.
CS: Combat Squad, one 10 man broken into two 5 man squads.
CSM: Chaos Space Marines
DAoT: Dark Age of Technology.  An age of the Imperium between 18k-23k 40k5 pg 122.
DAngels (DA): Dark Angels
Dark Harlistar:  Harlequin heavy Dark Eldar list.
DAVU, Dire Avengers as Vehicle Upgrade. Where for 60 points a min sized squad of troops inside a Falcon made it scoring (5th ed).  Credit: Spyrle
DC: Death Company, Blood Angels.
DBJ: Don't be a Jerk!  Tag we put on back of CONvergence badges.  Be cool, don't piss off fellow players thus maintaining potential gamers for future games.
DCA: Death Cult Assassins, Inquisitor Henchmen.
DCCW: Dreadnought Close Combat Weapon.
DE: Dark Eldar
Devs: Devastator Squad, Space Marines.
DH: DaemonHunters.  3rd ed Codex convering Grey Knights and Inquisitors
DK: Dreadknight, Grey Knights.
DKOK (DKK): Death Korps of Krieg, Imperial Guard..
DoA: Descent of Angels.  Ability of Blood Angels re-roll failed deep strike and scatter d6 instead of 2d6
DoC: Daemons of Chaos
DoM: Doom of Malan'tai, Tyranids
Doublewing: Dark Angels army list that contains both Deathwing and Ravenwing.
DoW = Dawn of War, another of the 3 mission types in the game.  Credit: Suijin.  5th ed.
DP: Daemon Prince.
DS: Deep Strike (40k6 p36)
DT: Designated Transport (40k6 p78), Dangerous Terrain (Mike from Fires of Battle GT)
DtW: Deny the Witch. (40k6 p68)
DV: Dark Vengence.  6th ed starter box.
DW: Draigowing, a Grey Knight army list.
DWK: Deathwing Knights, Dark Angels.
DW/RW: Deathwing and Ravenwing, Dark Angels.
DZ: Deployment Zone.
EADL: Eldar Aegis Defense Line.  (Keep up the good work Spyrle)
EL: Eldar
EML: Eldar Missile Launcher (40k6 p414)
FA: Fast Attack, Force Organisation Slot.
FAAC: Fashion at all Costs.  Building an army based on fashion not winning.  (This is mostly what I do, though I'm trying to balance the sides)
FAGK: Fast Attack Grey Knights
Fatecrusher: Chaos Daemons tactic of using Fateweaver's ability on Bloodcrushers.
FAQ: Frequently Asked Questions.
FC; Furious Charge, Special Rule 40k6 pg 37.
(F)LGS: (Favourite[Friendly]) Local Gaming Store.  The place where your wallet goes to die.  Friendly credited to Tasha.
FF: Flickerfield, Dark Eldar.
FoR: Fortress of Redemption.
Flyrant: Flying Hive Tyrant.
FMC = Flying Monstrous Creature.  Credit: Anonymous 12.7.31 10:35AM
FNP: Feel No Pain.  Special Rule (40k6 p35).  Ability to avoid being wounded.
FOC: Force Organization Chart
FRFSF : First Rank Fire, Second Rank Fire, Imperial Guard Order, if successful, unit's lasguns each gain one extra shot.  credited to Hudson
GEQ: Guard Equivalent: Lots of 5+ or worse saves (low stats)
GH: Grey Hunter, Space Wolves
GK: Grey Knights
GKSS: Grey Knights Strike Squad
GKT: Grey Knight Terminator.
GS: Green Stuff
GT (Grand Tournament): Regional event that usually consists of 2-3 days of 40k battles.
GtG: Go to Ground 40k6 pg 18
H2H: hand-to-hand
HF: Heavy Flamer.
HnA: Hammer and Anvil.  One of three deployments in 6th.
HP = Hull Points.  credit: Vargna
HtP: How to Paint a digital download.
IA: Imperial Armoury.
IC: Independent Character 40k6 pg 39
KFF: Kustom Forcefield, Orks.
KoS: Keeper of Secrets, Chaos Daemons.
KP: Kill Points.  Same as Victory Points.  q.v.VP
LC (LasC): Lascannon
LG: Liquifier Gun, Dark Eldar.
LoF: Lord of Fire, Necrons.
LOS: Line of Sight
LoS: Look Out, Sir. (40k6 p16).  Not to be confused with LOS.  Note lowercase "o".
LRC: Land Raider Crusader, Space Marines.
LRBT: Leman Russ Battle Tank, Imperial Guard.
LS: Land Speeder, Space Marines.
LSV: Land Speeder Variants.
Mathhammer:  Statistics people use to determine the result of dice rolled one unit vs another. q.v. (warning ESET anti-virus says something about the site having a Trojan and disables that tab, I use Firefox)
MC: Monstrous Creatures, Force Organisation Chart.
MEQ: Marine Equivalent: 3+ save (high stats)
Meta: Play style/army build.  Example: 40k Community in Jacksonville mostly use range armies.  40k Community in Dallas mostly use Hand-to-Hand armies.
MM: Multi-Melta
MoK: Mark of Khorne
MoO: Master of Ordinance, Imperial Guard.
MoN: Mark of Nurgle
MoS: Mark of Slaanesh
MoT: Mark of Tzeentch
MSS: Mind Shackle Scarabs, Necrons.
MSU: Multiple Small Units.  Elites, Troops, Fast Attack in squads less than 10.  Designed to spam the field and provide target saturation.  Target saturation provides a tremendous amount of units for your opponent to shoot at thus limiting the effectiveness of shooting at squads.
NDK: Nemesis Dread Knight, Grey Knights.
NE: Necrons
OoF: Officer of the Fleet, Imperial Guard.
PA: Power Armour
PAGK: Power Armour Grey Knights
Paladinstar: A Grey Knight army using Paladin's which are notoriously hard to kill.
PBS: Psyker Battle Squad, Imperial Guard.
PC: Psycannon i.e. Paladin 1 PC Banner.  Terminator Grey Knight with Psycannon and Banner.  Also Plasma Cannon.
PE: Preferred Enemy, Special Rule 40k6 pg 40.
PF: Powerfist
PGL: Phantom Grenade Launchers, Dark Eldar.
PotMS: Power of the Machine Spirit, Space Marines.
PS: Power Sword.
Psyback: Razorback with psyammunition, Grey Knights.
Psyrifle Dread (Psyflemen):  Grey Knights Dreadnought Autcannons with psyammunition.
PT: Personal Teleporter, Grey Knights.
Pulseteks: Crypteks with Solar Pulse, Necrons.
RAS: Ravenwing Attack Squadron.  Also Razorback Assault Squad.
RAW: Rules as Written.  Example: Mawlock is a large blast weapon therefore cannot hit flyers.
RB/HB: Razorback w/Heavy Bolter
RB/LP: Razorback w/Lascannon and Plasma gun
RCL: Random Charge Length
Rifledread: Dreadnought with Two Autocannons and psyammunition.  See: Psyflemen.
RoW: Runes of Warding, Eldar.
RJB: Reaver Jetbike, Dark Eldar.
RP: Rune Priest, Space Wolves.  Also Reanimation Protocols, 5th ed Necron rule.
RT: Rogue Trader, 1st ed 40k.
RTT: Rogue Trader Tournament.  Difference between Rogue Trader Tournaments and Grand Tournaments is that RTT's are one day tournaments.  GT's are multi-day tournaments.
RW: Ravenwing, Dark Angels.
SAFH: Shooty Army From Hell.  Typically referring to Imperial Guard because of the number of shots.
SB: Storm Bolter i.e. Paladin 1 SB Banner.  Terminator Grey Knight with Storm Bolter and Banner.
SC: Splinter Cannon, Dark Eldar.
SiTW: Shadow in the Warp, Tyranid.
SnP: Slow and Purposeful.  Special Rule (40k6 p42)
Soladin: Solo Paladin. Grey Knights
SoB = Sisters of Battle (AKA red-headed step children of the Imperium) Credit: Suijin.
SP: Shock Prow, Dark Eldar.
Sport: Short for Sportsmanship.  How one presents themselves during a game.
Sv: Save, have to roll equal to or greater than number on d6 to avoid being wounded.
SW: Space Wolves
S weave: Sempiternal Weave, Necrons
TA: Terminator Armour
TA Squad: Tactical Assault Squad, Space Marines.
TAC: Take all Comers, a list built to fight any army list.
Tabled: Every model has been removed from the table.
Tremorteks: Crypteks with Tremorstaves (Thanks IDICbeer)
TLAC: Twin-linked Assault Cannon
TLC: Two Lightning Claws
TLHvyB (TLHB) : Twin-linked Heavy Bolter
TLLC: Twin-linked Lascannon
TLPC: Twin-linked Plasma Cannon
That Guy: A gamer who is a rules-lawyer-*ssh#l@ and/or that is out to ruin other peoples fun.
tic tacs:  Little white breath tablets you use to win.
TEQ: Terminator Equivalent: 2+ save (high stats)
ToS: Tournament of Skulls.
TS: Temporal Snares, Necrons.
TH/SS: Thunder Hammer/Storm Shield, Space Marine.
USR: Universal Special Rules. i.e. Feel No Pain, Scout, etc. 5th ed.
VB: Venom Blade, Dark Eldar
Vindi: Vindicator, tank.
VoD: Veil of Darkness, Necrons.
VotLW: Veteran of the Long War, Chaos Space Marines.
VP: Victory Points. Example: Slay the Warlord Secondary Objective yields 1 VP, unless Purge the Alien then 2 VPs.
WAAC: Win at all Costs.  A method to abuse game mechanics to win the game.
WIP: Work in Progress, stages a model goes through when being put together and painted.
WLT: Warlord Traits.
WoC: Warriors of Chaos
WS: Warscythe, Necrons.
WW AA: Whirl Wind Anti-Aircraft, Space Marine.
WWB: We'll Be Back, 3rd ed Necron rule.
WWP: Webway Portal, Dark Eldar.