Table of Contents:
1. General
2. Chat
3. Dungeons
4. Raids
5. Group Roles
6. ICC bosses and links to Tankspot
7. Cities
8. Battle Grounds
9. Professions

Abbreviations will be used in the description of other abbreviations, just look further down the list for those definitions.
Google the locations, or have someone guide you if you cannot find them.

General:
GS- Gearscore: it is unfortunately the most used indicator of ÔÇ£skillÔÇØ, the higher GS, the more likely you can get into a group. You can see many debates about GS and how to ÔÇ£get around itÔÇØ in the forums but it is a hard fact in Wrath.

HC- Heroic Modes: Harder than normal in the same instance and may include slightly different tactics but give better gear than normal.
EOF- Emblem of Frost: you get these from ICC, RS, VOA, daily RDF HCs, weekly ÔÇ£must dieÔÇØ quests (usually just called ÔÇ£weeklysÔÇØ)
You use these emblems to buy some of the best gear from the venders in Dalaran. You can buy class sets from your class vendor and general items from the EOF vendor in your faction section of Dalaran.
Beware: these items are expensive and hard to get, so spend them wisely.
EOT- Emblem of Triumph: You get these from TOC raids, any ICC dungeon (the ones listed in the RDF), weeklys, and basically any instance in Northrend that does not give EOFs.
These are used to get T9 and EOT items. EOT vendors are usually located near the EOF vendors but class set vendors are the same. You can also get EOT items at the Ardent tournament.
T9/T10: Tier sets are items that go together that give you bonuses. T10 is bought with EOFs and T9 is bought with EOTs. To buy sanctified gear (which is one level above regular T10) you must have already bought the regular T10 and have won a mark from ICC 25 normal or ICC 10 HC
Buffs- Anything that is used on your character that gives a benefit.
Debuff- Anything used on your character that damages or is detrimental to it.
FFA- Free For All: rarely used in raids.
ML- Master Looter: used almost all the time in raids because it prevents people who don't need an item from winning it.
Group- Group Loot: everyone can roll, Similar to NBG
NBG- Need Before Greed: the default for RDFs

Chat
WTB- Want to Buy: used when someone wants to buy an item in global
WTS- Want to Sell: used when someone wants to sell an item in global
LFG- Looking for group: used to advertise yourself in global for a certain raid/dungeon/etc.
LFM- Looking for members: used when you need some people to fill your raid/dungeon group
AFK- Away from the Keyboard: exactly what it sounds like.
BRB- Be right back: Say this in a raid if you need to go get something so people don't start without you, or in general to let others know you are going afk.
imo- In my opinion: self explanitory

Dungeons:
FOS- Forge of Souls you need to go here with a quest that you can get from a person standing in front of your faction bank in Dalaran and complete the quest chain (which requires you to kill the bosses) before you can enter POS
POS- Pit of Sauron you will see this a lot in global for people farming it for emblems
HOR- Halls of Reflection: Most people donÔÇÖt ever do this because it is pretty hard to do and by the time most people can do it, they will be doing TOCs or ICCs.

Raids:
You can only do one of each raid on 10 man and one of each on 25 man per week. ie. You can only do one TOC 10 man, one TOC 10 man HC, one TOC 25 man and one TOC 25 man HC per week. You canÔÇÖt do two TOC 10 man normals in one week though.
10 mans consist of 10 people 25 mans consist of 25. 10m (10 man) are easier than 25m (25 man) but 25s have better gear.

ICC- the most common raid you will see in wrath. Don't expect to get in here before 5k GS in PUGs, and even then, that GS is low.
VOA- Vault of Archavon: The boss usually killed is just the Frost boss because it drops EOFs and t10, but sometimes a group will try to kill the fire boss which drops EOTs and t9.
RS- Ruby Sanctum: probably the hardest raid in Wrath. DonÔÇÖt expect to get in and down Halion (final boss) without having cleared ICC.
TOC- Trial of the Crusader: it is the raid you can do before you get into ICC and a good way to get your GS up. 25 mans contain recipes for some crafting professions.
OS- Obsidian Sanctum: very easy raid whose final boss, Sarth (Sartharion) is often the target of a weekly.
Naxx- Naxxramas: usually just done for fun or when a weekly requires a boss to be killed.
Ony- Onyxia's Lair: Not done too often, but done usually for the head, which is a quest that results in a relatively good item for tanks.

Roles:
DPS- Damage per Second: It means two things, the amount of damage you do per second according to recount or other similar meter addon and also the damaging role of the group.
There is the MDPS and the RDPS. MDPS is melee dps and RDPS is ranged dps.
Tank: keeps the boss and other mobs attacking it and not the DPS and healers.
There is the MT and OT. MT usually has better stats and is usually responsible for tanking the boss. OT usually picks up adds and is a backup in case the MT dies or something happens to the MT.
Healer: Does little to no damage and is responsible for keeping the group alive.

ICC Bosses (in order):
PLEASE look at the tankspot videos and learn how the fights work BEFORE you attempt to join an ICC.

Mobs- basically any regular enemy thatÔÇÖs not a boss
Adds- enemies that spawn during a boss fight that you usually have to kill
Marrow- Marrowgar
Lady- Lady Deathwisper
Saur/DBS- Death Bringer Saurfang
Gunship is not working
Rot- Rotface
Fester- Festergut
PP- Professor Putricide
BPC- Blood Prince Council
BQL/Queen: Blood Queen Lanathel
VDW- Valithra Dream Walker
Sindy- Sindragosa
LK- Lich King

Cities:

Alliance:
SW- Stormwind City: Most populous Alliance City
IF- Ironforge
Exodar- The Exodar
Darn- Darnassus

Horde: (I am not familiar with the Horde so please correct me if it's wrong)
Org- Orgrimmar: Most populous Horde city
UC- Undercity
SM- Silvermoon City
TB- Thunder Bluff

BGs- Battle Grounds: (I am not familiar with pvp so I don't have any info)

WSG - Warsong Gulch
EOTS - Eye of the Storm
AB - Arathi Basin
AV - Alterac Valley
IOC - Isle of Conquest
SOTA - Strand of the Ancients
WG - Wintergrasp

Crafting Professions (Proffs):
BS- Blacksmithing (not bull sh*t ): This profession crafts armor and weapons and some various other things.
JC- Jewelcrafting: makes gems for the sockets in your armor.
Alch- Alchemy: mainly used to transmute epic gems and make flasks that give a temporary boot to stats.
Ench- Enchanting: Enchants gear to give them a stat boost or effect.
Inscrip- Inscription: Makes glyphs for all classes and can also make some low level items.
Engineering- a pretty fun profession that gives some nice enchants and items that you can use
LW- Leather Working: Makes mail and Leather items along with some other stuff.
Tail- Tailoring: Makes cloth items and also some caster enchants

Gathering Professions:
Herb- Herbalism: Gives herbs used with Alchemy and Inscription
Mining- Gives ores which are smelted into bars used for Blacksmithing.
Skinning- Gives leather which is used for Leather Working.

Misc. Professions:
Fishing- Gives fish used for cooking.
Cooking- makes food that restores HP and Mana and can give a buff.
First Aid- used to make bandages.