Two styles of streaks can be found in Modern Warfare 2, Killstreaks give rewards for getting kills without dying, and Deathstreaks help players that can’t get kills with the ability to kill after death or can provide added bonuses like running faster after spawning.
At the start, the player can only use UAV, Care Package, and Predator Missile, with the option to unlock one new killstreak at each of the following levels: 10, 15, 23, 35, 47, 51, 55, 59, 63, 66, 68, and 69
Killstreaks

UAV
Shows enemies on the minimap.
Required: 3 Kills

Care Package
Airdrop a random killstreak or ammo.
Required: 4 Kills

Counter-UAV
Temporarily disables enemy radar.
Required: 4 Kills

Sentry Gun
Airdrop a placeable Sentry Gun.
Required: 5 Kills

Predator Missile
Remote control missile.
Required: 5 Kills

Precision Airstrike
Call in a directional airstrike.
Required: 6 Kills

Harrier Strike
Airstrike with a hovering Harrier.
Required: 7 Kills

Attack Helicopter
Call in a support helicopter.
Required: 7 Kills

Emergency Airdrop
Airdrop 4 random killstreaks or ammo.
Required: 8 Kills

Pave Low
Heavily armored assault helicopter.
Required: 9 Kills

Stealth Bomber
Airstrike undetectable on enemy maps.
Required: 9 Kills

Chopper Gunner
Be the gunner of an attack helicopter.
Required: 11 Kills

AC130
Be the gunner of an AC130.
Required: 11 Kills

EMP
Temporarily disables enemy electronics.
Required: 15 Kills

Tactical Nuke
End the game with a bang.
Required: 25 Kills
Deathstreaks

Copycat
Steal you killer’s class in kill cam.
Required: 4 deaths without a kill
Unlocked: Default

Juiced
Move faster for a few seconds after spawning.
Required: 3 deaths without a kill
Unlocked: Default

Martyrdom
Drop a live grenade just after dying.
Required: 4 deaths without a kill
Unlocked: Level 27

Final Stand
Get back up after being wounded.
Required: 4 deaths without a kill
Unlocked: Level 39