In the Call of Duty series, A Perk is an ability of sorts which can be equipped onto your player for use in multiplayer matches. There are 3 different perk slots (blue, red and green) which each have their own set of perks. E.g you can not equip a perk into whatever slot you like. The following is a description of all available perks. You unlock the Perks as you level up.
 Perk 1
Note that equipping a grenade launcher or grip onto one of your weapons will replace this perk slot.
Bomb Squad - Ability to seek out enemy explosives
C4 Explosives x2 - Remote Detonation explosive
Claymore x2 - Trip activated explosive mine
RPG-7 x2 - Rocket Launcher with 2 rockets
Special Grenades x3 - 3 special grenades. No Smoke
Frag x3 - 3 Frag Grenades
Bandolier - Extra Ammunition magazines
 Perk 2
Juggernaut - Increased Health
Sleight of hand - Faster Reloading
Stopping Power - Increased bullet damage
UAV Jammer - Undectable on enemy radar
Sonic Boom - Higher explosive weapon damage
Overkill - Carry two primary weapons, no pistol
Double Tap - Increased rate of fire
 Perk 3
Extreme Conditioning - Sprint for longer distances
Steady Aim - Increased hip-fire accuracy
Last Stand - Pull out your pistol before dying
Deep Impact - Deeper bullet penetration
Dead Silence Make less sound when you move
Iron Lungs - Longer breath for steadier sniper shots
Eavesdrop - Has been taken out.
Martyrdom - Drop a live grenade when killed.
 Kill Streak Perks
UAV - Calls in a UAV recon when 3 kill streak is achived.
Airstrike - Calls in an airstrike on a wanted position when 5 kill streak is achieved.
Helicopter - Calls in a Helicopter when 7 killstreak is achieved.