Abbreviations and Terms

Common slangs used ingame
Abbreviations
AD - Attack Damage
AP - Ability Power
CC - Crowd Control (stun, snare, slow, knockup etc.)
MIA - Missing In Action, someone is away from the lane
GJ - Good Job
GG - Good Game, usually said in /all chat when the game ends
OOM - Out Of Mana, this unit can't do much and might have to recall for more mana
TL, ML, BL - Top, Middle and Bottom lanes
Top, Mid, Bot - See above
DMG - Damage
Pen, MPen - Armour Penetration or Magic Penetration
CS - Creep Score or Creeps Slain
DPS - Damage Per Second
WP - Well played
CD - Cooldown
AS - Attack speed
LOM - Low on Mana
OMW - On my way

Terms
Squishy - Easy to kill, low health
Feeding - A poor player who has given the enemies lots of kills
Fed - Met up with a feeder
Combo - A champions standard combination of skills to deal damage
Gank - Surprise attack on your lane, most commonly by the enemy jungler
Warding - Usually the support or the jungler putting up wards so that the map is seen better
Skillshot - A skill that needs to be aimed
Juke - The act of moving side to side to dodge a skillshot
Creep - A minion
Neutral Creep - Neutral jungle minions
Laning Phase - Early game, everyone's laning
Counter Jungle - Invading the enemy jungle to somehow damage the enemy jungler
Aura - Something that gives an effect to those around it.

Now go call the shots!