Perk Slot 1
Recon / Recon Pro

Every enemy that takes explosive damage is marked as a red arrow on the radar, which also shows the direction the enemy is facing.

Pro Ability: Same as above, but it's achieved by causing bullet damage to the enemy.
Sleight of Hand / Sleight of Hand Pro

Reload your weapons 50% faster than normal. Sleight of Hand is extremely useful with Light Machine Guns since they have long reload times.

Pro Ability: Pick up and swap weapons 50% faster than normal.
Blind Eye / Blind Eye Pro

Enemy air support and sentries can't detect you.

Pro Ability: Faster launcher lock-on + extra bullet damage vs. enemy air support. This makes Light Machine Guns extremely effective against enemy air support.
Extreme Conditioning / Extreme Conditioning Pro

Allows you to sprint 2x times longer. Press and hold the left analog stick on the Xbox 360 (LS) or PlayStation 3 (L3) to sprint.

Extreme Conditioning is useful for aggressive rush tactics.

Pro Ability: Climb obstacles faster.
Scavenger / Scavenger Pro

Replenishes your ammunition when you walk over Scavenger bags that are dropped by players when they die. Grenade launcher ammo can only be replenished if the player that dropped the Scavenger bag had the same ammo.

Pro Ability: Extra starting magazines.

Perk Slot 2
Quickdraw / Quickdraw Pro

Allows you to aim down the sight 50% faster than normal. Quickdraw is ideal for quick-scoping with Sniper Rifles.

Pro Ability: Allows you to recover from equipment and grenade usage faster.
Blast Shield / Blast Shield Pro

Greatly reduces explosive damage.

Pro Ability: Makes you immune to Stun and Flash Grenades.
Hardline / Hardline Pro

Reduces the amount of kills required for any killstreak by one. For example, the Care Package normally requires 4 kills, but with Hardline, you only need 3 kills.

Pro Ability: Two kill assists add one kill towards your next killstreak. Deathstreaks require one less death.
Assassin / Assassin Pro

Makes you undetectable by UAV, thermal scopes, Heartbeat Sensors and Portable Radars. You can still show up on the radar if you fire a weapon that doesn't have a silencer.

Assassin doesn't protect you from the Recon perk or the Recon Drone killstreak.

Pro Ability: Makes you immune to Counter-UAV and EMP. Enemies won't see a red crosshair or name when targeting you.
Overkill / Overkill Pro

Allows you to carry two primary weapons, but the second primary weapon can't be equipped with attachments.

Pro Ability: Your second primary weapon can now be equipped with attachments.

Perk Slot 3
Marksman / Marksman Pro

Allows you to identify enemies from farther away by showing their names at a greater distance.

Pro Ability: Allows you to hold your breath 2x times longer than normal when steadying a weapons scope.
Stalker / Stalker Pro

Faster movement while aiming down the sight.

Pro Ability: Delays enemy detection-triggered explosives, like Claymores and Bouncing Betties.
Sitrep / Sitrep Pro

Highlights enemy explosives and tactical equipment in red.

Pro Ability: Makes enemy footsteps louder and also cancels out Dead Silence.
Steady Aim / Steady Aim Pro

Increases hip-fire accuracy.

Pro Ability: Allows you to aim down the sight faster after sprinting.
Dead Silence / Dead Silence Pro

Makes you silent and also reduces the effect of the Recon perk.

Pro Ability: You don't take any damage from falling.