Flak Jacket

Flak Jacket is a Tier 2 perk featured in Call of Duty: World at War and a Tier 1 perk that will be featured in Call of Duty: Black Ops, that reduces explosive damage done to the player.

Call of Duty: World at War
In World At War, it is unlocked at rank one in the "Heavy Gunner" class.

Flak Jacket decreases all explosive damage taken by 25%, and caps the maximum damage taken from any one explosive (excluding a direct hit from a grenade launcher, Bazooka or a tank shell) to 75. In effect, this allows the player to survive any single indirect explosive attack. Players who find themselves constantly killed by Grenades, Bouncing Betties, Tanks, Satchels, Artillery Strikes or M9A1 Bazooka will find this perk to be very useful. This perk has very little effect in Hardcore, due to decreasing HP of players, since the 75 damage cap is far above the maximum health level.

Tips
This perk is commonly used in Headquarters, Domination and War to prevent being killed by grenades while trying to capture a flag or HQ. It is also especially useful in Search and Destroy, as the enemy will plant Bouncing Betties on the target to kill the player planting the bomb. It can also be helpful when camping in the watchtower on Makin because players will often throw grenades into the tower in hopes of killing the player. It is often used with SMGs or Shotguns, as they can deal high damage even without perks such as Stopping Power.

Call of Duty: Black Ops
Flak Jacket will return as a tier 1 perk in Black Ops.

Flak Jacket will reduce explosive damage received by the player in a very similar way to World at War. The explosive damage is not only reduced but capped at 75% of the player's maximum total health allowing the player to survive almost all explosive attacks. The pro version of this perk will give the player the Toss Back and Fireproof perk abilities from World at War. Players using the Flak Jacket perk will wear significant amounts of armor

Trivia

 * The Blast Shield in Modern Warfare 2, when equipped obscures the player's view and also protects the player against Stun Grenades whereas in Call of Duty: World at War Flak jacket doesn't.
 * Although very helpful for surviving indirect explosions, if someone attaches a Sticky Grenade to them, the player will still die.
 * Elements of this perk are survived in the Blast Shield, a piece of equipment in Call of Duty: Modern Warfare 2.
 * In the South Park episode "The Ungroundable", everyone in the computer room is playing Call of Duty: World at War. Stanley Marsh gets angry for not killing a classmate with an explosive because of the Flak Jacket perk, shouting "Ok, who's using the Flak Jacket cheat?", and his classmate answers "Not a cheat if you rank up!"