Well, as far as WoW specifically goes you have (in no particular order):
agg/aggro: Having the attention of a specific mob, usually having them focus their attacks on you.
dps: Damage Per Second, a method of measuring damage output a character/mob can produce
dot: Damage Over Time. Refers to an ability that deals damage at regular intervals instead of all at once.
hot: Heal Over Time. Refers to an ability that heals damage at regular intervals instead of all at once.
tick: Period of time, usually the smallest period of time that something occurs in. For example: "My dot does 300 damage a tick."
mob: Enemy character or monster.
pull: To get the attention of a mob a distance away and cause them to move to your vicinity, usually so as to avoid other mob's in the same area while you take the first one down.
pat: Short for patrol. Used to reference a mob or group of mobs that move in a set pattern from one area to another.
tank: To keep the attention of a mob focused squarely on you while the rest of your party deals as much damage to it as possible. Good tanks need to keep aggro and be able to take large amounts of damage without dying.
pots: Short for potions. Potions are used to regain heath/mana or give players an ability for a short period of time.
trash: Refers to various mobs that stand between you and the boss.
vendor trash: Refers to items (often received from trash mobs) that's purpose is solely to be sold for cash, or items that are always worse than anything else of its level.
loot: Items received from defeating mobs.
adds: Mobs that another mob (Usually a boss mob) can create/call to distract the party from killing it.
pet: A secondary creature that is under player control, usually in the form of a lower level mob.
vanity pet: A pet that has no in game use other than visually. It gives no advantage to the player that has it.
faction: A group or organization within the game.
favor: The points a player has with a faction that determine how said faction treats the player.
gear: Equipment used by a player.
PVP: Player Versus Player. Refers to instances or areas where players can play in direct competition with other players.
PVE: Player Versus Environment. Refers to instances where players cannot play in direct competition with other players, but instead work together against the standard enemies in the game.
stealth: The ability to go undetected by enemies.
epics: The top level of gear available in the game (at least within it's level range).
purples: Epics
blues: High ranked magical items (within it's level range).
greens: Low ranked magical items (within it's level range).
whites: Items with no magical enhancements.
spec: A characters spec is the way the character is customized, including which talents and abilities the character has. Usually defined in broad categories (for example feral spec).
talent: Abilities that affect a characters abilities, how often they can use them, how effective they are, etc.
unique: An item a character can only have one of at a time. Other characters may also have the same item, but each character can only have one of.
group: A party of characters, usually a maximum of 5, that work together to accomplish a goal. Grouped characters can see each others health and mana, can target each other with beneficial spells or abilities, and have a separate chat so as to better help one another.
instance: Refers to a specific area only accessible to you and the group you are in. Other players/groups can enter the area, but it will load specific to them. They will not see other players in the same area but a different instance, and any mobs they kill or things they affect will only be affected in their instance and not in another player's instance.
dungeon: A building or otherwise enclosed area filled with mobs, often having a boss mob at the end.
raid: A dungeon or other area that allows a larger than standard group to enter and work together to fight a much higher level boss. May also refer to the group itself. Examples: 10 man raid, 25 man raid, (insert end boss name here) raid.
OOM: Out Of Mana. Used by a spell casting class to alert others in the group that they are out of mana and will be doing less damage/unable to heal players until their mana regenerates.
cooldown: The period of time immediately after an ability is used when it or other abilities is unable to be used again. Example: This spell does great damage, but has a 5 minute cooldown.
res: Short for resurrect. Used for any ability that brings a character back to life after they are killed.
lfg: Looking For Group
lfm: Looking For More or Looking For Member. Used when a group needs additional people before doing a quest.
wtb: Want To Buy
wts: Want To Sell
wtt: Want To Trade
afk: Away From Keyboard. Used to alert other party members you will be inactive for a period of time.
bio break: More specific form of AFK. Used to alert other party members you have to use the bathroom.
brb: Be Right Back. Used when a player will be afk, but only for a few seconds. For example, when they need to answer a question from someone in the room with them.
That is a lot of them. I'll add more as I think of them.
) and leet.
