Perks are a part of the Call of Duty online experience. They act as your power-ups and provide you with a bunch of cool stuff.

One upgrade to the perks system is that now each of them can be upgraded to a "Pro" rating.

Perk Slot 1

Marathon | Marathon Pro

• Ability: Unlimited sprint.
• Pro Ability: Get over obstacles faster.

Sleight of Hand | Sleight of Hand Pro

• Ability: Faster reloading.
• Pro Ability: Gives a speed boost to how fast you can aim down the sight of your weapon.

Scavenger | Scavenger Pro

• Ability: Full resupply from dead enemies.
• Pro Ability: Extra magazines. 

Bling | Bling Pro

• Ability: 2 primary weapon attachments.
• Pro Ability: 2 secondary weapon attachments.

One Man Army | One Man Army Pro

• Ability: Replaces your secondary weapon. This allows you to change your class at any given time during a match.
• Pro Ability: Swaps classes even faster. 

Perk Slot 2

Stopping Power | Stopping Power Pro

• Ability: Increased bullet damage.
• Pro Ability: Increased bullet damage vs. helicopters and aircrafts. 

Lightweight | Lightweight Pro

• Ability: Move faster.
• Pro Ability: Allows you to do a quick aim after sprinting. 

Hardline | Hardline Pro

• Ability: Killstreaks require 1 less kill.
• Pro Ability: Death Streaks require 1 less death. 

Cold-Blooded | Cold-Blooded Pro

• Ability: Undetectable by UAV, air support, Sentry Guns, and thermal.
• Pro Ability: No red crosshair or name shown when you are targeted. 

Danger Close | Danger Close Pro

• Ability: Increases explosive weapons damage.
• Pro Ability: Increases air support damage.

Perk Slot 3

Commando | Commando Pro

• Ability: Increased melee distance.
• Pro Ability: You don't take any damage from falling. 

Steady Aim | Steady Aim Pro

• Ability: Increased hip-fire accuracy.
• Pro Ability: You can hold your breath for a longer period of time. 

Scrambler | Scrambler Pro

• Ability: Scrambles (jams) the enemy radar in a small radius.
• Pro Ability: Delays enemy Claymores, thus allowing you to avoid the explosion if you are fast enough. 

Ninja | Ninja Pro

• Ability: Invisible to heartbeat sensors.
• Pro Ability: Allows you to move silently. 

SitRep | SitRep Pro

• Ability: Insertions. This will reveal enemy explosives and Tactical Insertions.
• Pro Ability: Makes enemy footsteps louder. 

Last Stand | Last Stand Pro

• Ability: Allows you to survive a normal death. You will drop to the ground and pull out your pistol. Last Stand ends after ~10 seconds or if you get shot again, which results in your death. You can also crawl in the Last Stand mode.
• Pro Ability: You get to use your equipment too, like frag grenades and throwing knives. 

Two things i noticed, that personally bring a smile to my face... No more "Martyrdom" and no more "Second chance".

Source - TMW2.com