Perks

From the Modern Warfare 2 Wiki
Perks (referred as Specialties) are multiplayer modifiers in Call of Duty 4: Modern Warfare, Call of Duty: World at War, Call of Duty: Modern Warfare 2, Call of Duty: Black Ops and Call of Duty: Black Ops (Nintendo DS) that passively affect game play, allowing further customization of classes. They affect available equipment, effectiveness of the player's equipment, the usefulness of the enemy's equipment, and the physical properties of the player's character.

Call of Duty: Modern Warfare 2 Perks
In Call of Duty: Modern Warfare 2 the perks are divided into tiers in different way than in Call of Duty 4 and World at War Along with standard perks, players will now be able to unlock improved "Pro" versions of certain perks by fulfilling certain requirements while using them. These "Pro" versions include the original abilities of the perk, but also add new benefits. Many of the perks that were in Call of Duty 4 or 5 that are missing are now either part of a Pro perk, available as weapon attachments, or are now deathstreaks, such as Deep Impact (FMJ), Martyrdom and Double Tap (Rapid Fire). The Overkill perk has been replaced partially by the inclusion of Machine Pistols, Shotguns, and Launchers as secondary weapons and also by the One Man Army and Bling perks. Interestingly, if a Perk Challenge IV is achieved mid-game the "Pro" version will take effect immediately for all classes equipped with that perk. However, the effect of the Pro version is granted after respawning. Perks from Modern Warfare 2 Perks from Modern Warfare 2

First Tier
This tier affects players in terms of Movement Speed (Sleight of Hand/Marathon), Ammunition (Scavenger) and Equipment selection (Bling/One Man Army).

Marathon (Unlocked at level 1) Sleight of Hand (Unlocked at level 1)
 * Unlimited sprint. (Replaces Extreme Conditioning)300px-MW2_Perks.jpg
 * Marathon Pro: Climb ladders and objects faster.
 * Unlock Pro: Run 26 miles using marathon.
 * Unlock Emblem: Run 104 miles using marathon

Faster reloading. Scavenger (Unlocked at level 13) Bling (Unlocked at level 21) One Man Army (Unlocked at level 45)
 * Sleight of Hand Pro: Faster aiming down the sight or scope.
 * Unlock Pro: Get 120 kills using Sleight of Hand.
 * Unlock Emblem: Get 750 kills using Sleight of Hand.
 * Resupply from dead bodies.
 * Scavenger Pro: Extra mags upon spawning.
 * Unlock Pro: Resupply from 100 dead bodies.
 * Unlock Emblem: Resupply from 500 dead bodies.
 * Allows two attachments to primary weapon.
 * Bling Pro: Allows two attachments to secondary weapon.
 * Unlock Pro: Get 200 kills using two weapon attachments
 * Unlock Emblem: Get 900 kills using two weapon attachments.
 * Allows class switching ingame. (Replaces secondary weapon)
 * One Man Army Pro: Faster class switching (from 5 seconds to 3).
 * Unlock Pro: Get 120 kills using One Man Army
 * Unlock Emblem: Get 750 kills using One Man Army

Second Tier
This Tier deals with the effects on Damage (Stopping Power/Danger Close), Movement (Lightweight) and bonus effects on Killstreaks (Hardline/Cold-Blooded).

Stopping Power (Unlocked at level 1) Lightweight (Unlocked at level 1) Hardline (Unlocked at level 9) Cold-Blooded (Unlocked at level 25) Danger Close (Unlocked at level 33)
 * Increased bullet damage by 40%.
 * Stopping Power Pro: Increased damage versus vehicles.
 * Unlock Pro: Get 250 kills using Stopping Power
 * Unlock Emblem: Get 1000 kills using Stopping Power
 * Move 7% faster.
 * Lightweight Pro: Quicker aiming after sprinting.
 * Unlock Pro: Run 30 miles using Lightweight.
 * Unlock Emblem: Run 250 miles using Lightweight.
 * Killstreaks require 1 less kill.
 * Hardline Pro: Deathstreaks require 1 less death.
 * Unlock Pro: Get 40 killstreaks while using Hardline.
 * Unlock Emblem: Get 250 killstreaks while using Hardline.
 * Makes the user invisible to enemy thermal optics, and enemy killstreaks. (replaces UAV Jammer and Camouflage)
 * Cold-Blooded Pro: No red crosshair or name when targeted.
 * Unlock Pro: Destroy 40 killstreak rewards using Cold Blooded.
 * Unlock Emblem: Destroy 250 killstreak rewards using Cold-Blooded.
 * Increased explosive damage. (replaces Sonic Boom and Fireworks)
 * Danger Close Pro: Increased damage from killstreak rewards.
 * Unlock Pro: 100 kills using explosives while using Danger Close.
 * Unlock Emblem: 500 kills using explosives while using Danger Close.

Third Tier
This tier effects the enemy players equipment (Scrambler/Ninja/Sitrep)and gives players a tactical advantage in combat (Steady Aim/Commando/Last Stand).

Commando (Unlocked at level 1) Steady Aim (Unlocked at level 1) Scrambler (Unlocked at level 17) Ninja (Unlocked at level 29) SitRep (Unlocked at level 37) Last Stand (Unlocked at level 41)
 * Increased melee lunge distance.
 * Commando Pro: No falling damage.
 * Unlock Pro: 20 melee kills using Commando
 * Unlock Emblem: 100 melee kills using Commando
 * Increased hip-fire accuracy.
 * Steady Aim Pro: Breath can be held longer while aiming down a sniper or thermal scope. (Replaces Iron Lungs)
 * Unlock Pro: 80 hip fire kills using Steady Aim.
 * Unlock Emblem: 500 hip-fire kills using Steady Aim.
 * Jams the enemy radar when they are close enough.
 * Scrambler Pro: Delays enemy claymore explosions.
 * Unlock Pro: 50 close range kills using Scrambler.
 * Unlock Emblem: 250 close range khttp://modernwarfareii.wikia.com/index.php?title=Perks&action=editills using Scrambler.
 * Makes Player invisible to the Heartbeat Sensor.
 * Ninja Pro: 4x quieter footsteps. (Replaces Dead Silence)
 * Unlock Pro: 50 close range kills using Ninja.
 * Unlock Emblem: 250 close range kills using Ninja.
 * Detect enemy explosives (Claymores, C4, and Grenades) and Tactical Insertions. (Replaces Bomb Squad)
 * SitRep Pro: 4x louder enemy footsteps (Cancels out Ninja Pro users, whose footsteps remain at 1/4x regular volume).
 * Unlock Pro: Destroy 120 enemy explosives using Sitrep.
 * Unlock Emblem: Destroy 750 enemy explosives using Sitrep.
 * Use a pistol while bleeding out prior to death.
 * Last Stand Pro: Use your equipment and knife in Last Stand.
 * Unlock Pro: 20 kills while in Last Stand
 * Unlock Emblem: 100 kills while in Last Stand