There's a great deal of information about 40k all over the web.  Many of it uses short hand to communicate.  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.

Tyranid Codex 2nd ed

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
AT: Anti-transport
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.
BC: Bloodcrushers, Chaos Daemons.
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
BFS: Battle for Salvation.  Grand Tournament held in NY.
BGB: Basic Game Book.
BL: Bloodletters, Chaos Daemons.
BLOS terrain: Blocking Line of Sight terrain
BotBG: Blessing of the Blood God, Khorne Gift
BP: Bolt Pistol
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
BWG: Beltway Gamers.  A gaming community in Washington. 
BYB: Big Yellow Book.  6th ed corebook.
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.
CotS: Chooser of the Slain.  Space Wolves wargear.
CS: Combat Squad, one 10 man broken into two 5 man squads.
CSM: Chaos Space Marines
DAngels (DA): Dark Angels
DAoT: Dark Age of Technology.  An age of the Imperium between 18k-23k 40k5 pg 122.
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.
DftS: Death from the Skies.  40k Supplement book covering rules for Fliers.
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.
DW/RW: Deathwing and Ravenwing, Dark Angels.
DWK: Deathwing Knights, 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(s): 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
hammenators: Terminators with Thunder Hammers.  Space Marine.
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
Instant Death: If weapon strength doubles the toughness of the model the model is removed from play regardless of the amount of wounds it has.  40k6 pg 16.  Eternal Warrior ignores this special rule
IW: Iron Warriors, Chaos Space Marines.
KFF: Kustom Forcefield, Orks.
KoS: Keeper of Secrets, Chaos Daemons.
KP: Kill Points.  Same as Victory Points.  q.v.VP
LC (LasC): Lascannon
LF: Long Fang, Space Wolves.
LG: Liquifier Gun, Dark Eldar.
LL: Living Lightning, Space Wolves psychic power.
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.
ML: Missile Launcher
MM: Multi-Melta
MoK: Mark of Khorne
MoN: Mark of Nurgle
MoO: Master of Ordinance, Imperial Guard.
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 Dreadknight, 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.
PCS: Platoon Command Squad, Imperial Guard.
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.
RJB: Reaver Jetbike, Dark Eldar.
RoW: Runes of Warding, 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.
SG: Soul Grinder, Chaos Daemons.
SiTW: Shadow in the Warp, Tyranid.
SnP: Slow and Purposeful.  Special Rule (40k6 p42)
SoB = Sisters of Battle (AKA red-headed step children of the Imperium) Credit: Suijin.
Soladin: Solo Paladin. Grey Knights
SP: Shock Prow, Dark Eldar.
Sport: Short for Sportsmanship.  How one presents themselves during a game.
SS/CF: Storm Shield/Chainfist, Space Marines.
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.
Tabled: Every model has been removed from the table.
TAC: Take all Comers, a list built to fight any army list.
TEQ: Terminator Equivalent: 2+ save (high stats)
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.
TH/SS: Thunder Hammer/Storm Shield, Space Marine.
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
TML: Typhoon Missile Launcher. ToS: Tournament of Skulls.
TS: Temporal Snares, Necrons.
TWC: Thunderwolf Cavalry, Space Wolves.
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.
WGBL: Wolf Guard Battle Leader, Space Wolves.
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.