Killstreak Rewards are bonuses that can be earned by killing a certain amount of enemies in a row without dying. You can lower the required amount by 1 with the Hardline perk.

Modern Warfare 2 expands the killstreak system of the Call of Duty Series. You have 3 killstreak rewards that you can customize to your liking. You start off with the UAV, Care Package and Predator Missile. As you reach level 10 and beyond, you earn points which you can use to unlock the rest of the killstreaks in any order you choose.

Killstreaks stack up, meaning that you don't lose a UAV after gaining, for example, a Predator Missile. Stacked up killstreaks can't be used as pleased, instead they need to be used in the order you've acquired them.

Previously, Killstreaks in Game Modes like Search & Destroy Your K-D wouldn't carry over from round to round.(ex. You get two Kills in the first round and never die then get another in the next round, but you still Don't get a UAV.) Now, In Modern Warfare 2, YOur Killstreaks carry over so if the example above^ You would get your UAV.

All killstreak killscount towards your next killstreak, except for those that have been acquired from Care Packages. All killstreaks that have been earned during previous lives will not add to your total.


# 3 Kills
UAV
Description: This deploys a UAV (unmanned aerial vehicle) that shows enemies on the radar for 30 seconds. Multiple UAVs (from the same team) makes the refresh rate faster.

Unlike in Call of Duty 4, UAVs fly around the map, so they can be shot down. They are rather weak, so it's better to use your primary weapon rather than wasting a valuable missile on them. Players using the Cold-Blooded perk won't show up on the radar.
# 4 Kills
Care Package
Description: Airdrops a random killstreak or ammo. If you can't get to the high level killstreaks on your own, use this. Just throw the red signal flare and wait for the helicopter to drop the package. The package can be stolen by anyone, even by your team mates, so be careful!

Care Packages can be seen on the radar by everyone. It's marked green if it's friendly, otherwise red since it belongs to the enemy team. Care Packages, Emergency Airdrops and Tactical Nukes can't be acquired from the package. Below are the drop rates:

14.78% - Ammo
14.78% - UAV
13.04% - Counter-UAV
10.43% - Sentry Gun
10.43% - Predator Missile
9.57% - Precision Airstrike
6.09% - Harrier Strike
6.09% - Attack Helicopter
4.35% - Pave Low
4.35% - Stealth Bomber
2.61% - Chopper Gunner
2.61% - AC-130
0.87% - EMP
Counter-UAV
Description: Scrambles the enemy radar for a short period of time, even if they don't have a UAV in the air.

Counter-UAVs fly around the map, so they can be shot down just like regular UAVs.
# 5 Kills
Sentry Gun
Description: Airdrop a placeable XM312 Sentry Gun. A helicopter will drop the weapon, which you can pick up and place anywhere you want. The Sentry Gun (box) can be stolen by anyone, just like the Care Package, so be sure to pick it up straight away. You can move the Sentry Gun around even after you've deployed it, but it won't fire until it's placed.

The Sentry Gun has a 180 degree turn on its turret and can pitch up and down, but it can't detect players using the Cold-Blooded perk. They are most effective when placed in high places.

Sentry Guns can be temporarily disabled with Flashbangs and Stun Grenades. Normal weapons don't do much damage to it, so use explosives instead. You can instantly destroy them with a melee hit or a Throwing Knife - silly, but true. The turret can over-heat if it's fired for too long, so use that to your advantage. The Sentry Gun will disable itself after 90 seconds.
Predator Missile
Description: Allows you to remote control a Predator Missile to the ground. Your screen will change to an overhead view and you have approximately 5 seconds to control the missile to your target. You can give the missile a boost to make it go down faster.

Enemies that don't have the Cold-Blooded perk will appear with a red box around them. The Predator Missile has a moderate explosion radius, so you don't need to hit the enemy dead-on. You can also use it to take down enemy helicopters and harriers.

You are vulnerable while controlling the Predator Missile, but if you get killed on the ground, you can still continue your missile attack.
# 6 Kills
Precision Airstrike
Description: This is much like the Airstrike from Call of Duty 4, but this time you can control the direction of the Airstrike for more accurate kills.
# 7 Kills
Harrier Strike
Description: Airstrike with a hovering harrier. The selected area gets bombed, after that a hovering harrier will guard the area for 45 seconds and tries to kill all nearby enemies. It can also attack enemy helicopters.

This basically combines the Airstrike and Attack Helicopter to one neat package, but the harrier doesn't circle the map, it just stays still. Harriers can be destroyed with one stinger/javelin missile.
Attack Helicopter
Description: Deploys a Attack Helicopter to the map, just like in Call of Duty 4. It will circle around the map and automatically attack enemies for 60 seconds.

Attack Helicopters can be destroyed with one stinger/javelin missile.
# 8 Kills
Emergency Airdrop
Description: Calls in a cargo plane that drops 4 Care Packages.

Try to avoid situations where the packages land outside the map. The plane always comes from the opposite direction you're facing (when throwing the red signal flare) and flies in a straight line.

Care Packages, Emergency Airdrops, EMPs and Tactical Nukes can't be acquired from the packages. Below are the drop rates:

12% - Ammo
12% - UAV
16% - Counter-UAV
16% - Sentry Gun
14% - Predator Missile
10% - Precision Airstrike
5% - Harrier Strike
5% - Attack Helicopter
3% - Pave Low
3% - Stealth Bomber
2% - Chopper Gunner
2% - AC-130
# 9 Kills
Pave Low
Description: Calls in a heavily armored attack helicopter (Sikorsky MH-53 Pave Low) to patrol the map for 60 seconds. It works just like the Attack Helicopter, but it has twice the armor and two turrets.

The Pave Low also has chaff (1 set) as a countermeasure, so the first locked-on missile won't hit it.
Stealth Bomber
Description: Calls in a Stealth Bomber (B-2 Spirit) to carpet bomb the selected area. You can also control the direction, just like with the Precision Airstrike.

The airstrike doesn't show on enemy radar.
# 11 Kills
Chopper Gunner
Description: Be the gunner of an attack helicopter. Your screen changes to the gunners view and the helicopter will fly around the map. The helicopter flies around for 40 seconds before it leaves, which does not include the time it takes to travel to the map.

The 25mm gun doesn't need to be reloaded, so you can easily kill multiple enemies. Enemies that don't have the Cold-Blooded perk will appear with a red box around them. You are also vulnerable while using this killstreak, but if you get killed on the ground, you can still continue your session.

The helicopter has 1 set of flares, so the first locked-on missile won't hit it.
AC-130
Description: Be the gunner of an AC-130 Gunship. Your screen changes to the gunners view and the AC-130 will fly around the map. You have 40 seconds to use the 25mm, 45mm and 105mm guns to shoot at enemies, but you can also take down enemy helicopters and harriers.

Enemies that don't have the Cold-Blooded perk will appear with a red box around them. You are vulnerable while controlling the AC-130, but if you get killed on the ground, you can still continue your session.

The AC-130 can only be shot down with missiles, but it has 2 sets of flares as countermeasures, so the first 2 missiles won't hit it.
# 15 Kills
EMP
Description: The electromagnetic pulse (EMP) temporarily disables enemy electronics for 60 seconds. It also stops the enemy from calling in killstreaks, and at the same time drops down any helicopters and aircrafts that are in the air.

The EMP also has the following effects on the enemy:
Removes the heads-up display, HUD.
Disables electronic sights.
Red Dot Sight, Holographic Sight, Thermal Scope etc.
Heartbeat Sensors won't work.
Disables Sentry Guns.
# 25 Kills
Tactical Nuke
Description: Calls in a nuclear strike that kills everyone on the map. It gives you and your team an instant win.

Once activated, nothing can stop it.


Source:http://www.themodernwarfare2.com/mw2/multiplayer/killstreaks/
Percentages were both taken of the website. And tested by me to make sure they were legit