From the Modern Warfare 2 Wiki\n\n 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.
There are eight perk packages in total that you can select and equip to your Loadout in Modern Warfare 2. Each perk package comes with two Basic Perks, one Bonus Perk, and one Ultimate Perk. Some perks can be found in more than one perk package, but each perk package as a unique combination of perks you can equip.