AFK - Away from keyboard, sometimes people use this term as if someone is not here or moving or doing anything.
Aggro - The tower is aiming and attacking you since you're in the opponents turrent range where it can cause damage to you.
AoE - Area of Effect.
B - To retreat or to recall back to the fountain to regain full health and mana and to buy items if possible.
Build – The items that you buy for your hero to fight. (Like boots, wand, etc)
CC - Crowd Control (stun, snare, slow, knockup etc.)
MIA - Missing In Action, someone is away from the lane
SS - "Miss", EU version of MIA
GJ - Good Job
GG - Good Game, usually said in /all chat when the game ends
TL, ML, BL - Top, Middle and Bottom lanes
Top, Mid, Bot - See above
DMG - Damage
CS - Creep Score or Creeps Slain
DPS - Damage Per Second
WP - Well played
CD - Cooldown
AS - Attack speed
N1 - Nice one.
MS - Movement Speed
Passive – Your "ability" that is activated (on at all times). Some passives have CD.