In Black Ops 2 multiplayer Killstreaks have been replaced with Scorestreaks, which are earned by killing enemies, assisting teammates and completing different objectives. For example, a normal kill gets you 100 points, but capturing a flag gives you 200 points - you will also get points when your teammates kill enemies while your UAV is active.
The UAV, for example, can be called in at 350 points, but you will need 1600 points to call in the powerful VTOL Warship. You can equip a maximum of 3 Scorestreak rewards at the same time.
Your Scorestreak resets to zero once you die. The equipped Scorestreak rewards in BO2 can be earned multiple times in one life, but they need to be used before the Scorestreak loops around for the second time or each Scorestreak reward is replaced / wasted with the same reward again.
UAV - 350 Points
Shows enemies on the radar as red dots for 30 seconds. The UAV can be shot down by the enemy.
RC-XD - 450 Points
Allows you to remote control a small car that's packed with explosives. The RC-XD can be detonated at will - enemies can also shoot to destroy it.
Hunter Killer - 525 Points
Deploys a flying drone that seeks out and tries to destroy the nearest enemy player or vehicle.
Care Package - 550 Points
The Care Package is the same as in previous Call of Duty games: it gives you a random scorestreak / killstreak. The Care Package can be hacked by the enemy team.
Counter-UAV - 600 Points
Temporarily disables the enemy radar for 30 seconds - the Counter-UAV drone can be shot down by the enemy.
Guardian - 650 Points
After the Guardian dish is deployed, it projects a microwave field that stuns enemies by slowing them down and also making their vision blurry.
Hellstorm Missile - 700 Points
The Hellstorm Missile works like the Predator Missile from the Modern Warfare series; it allows you to remotely control one missile to the ground. The missile can be scattered into a cluster bomb at will.
Lightning Strike - 750 Points
Equivalent to the Mortar Team killstreak from Call of Duty: Black Ops. You can choose three locations on the map for a coordinated strike.
Sentry Gun - 800 Points
An advanced version of the normal Sentry Gun that's been featured in previous Call of Duty games; once deployed, the Sentry Gun automatically attacks enemies. This version can, however, be remote-controlled at anytime.
Death Machine - 850 Points
Gives you a powerful minigun that can take out enemies with a couple of hits. Unlike in Black Ops, the Death Machine lasts until it runs out of ammo, even after dying - the Scorestreak can be reactivated and the ammo count is where you left off. All killstreak weapons work the same way on the Xbox 360, PlayStation 3, Wii U and PC versions.
War Machine - 900 Points
The War Machine is a semi-automatic grenade launcher with a 6 round magazine. The War Machine works like the Death Machine; it lasts until all the grenades have been used. This killstreak has been removed from the scorestreaks in black ops 3 and is instead a BO3 Specialist weapon with similar characteristics.
Dragonfire - 975 Points
Allows you to remote-control a quadrotor droid that's equipped with a lightweight machine gun. The Dragonfire is lightly armored, so it can be shot down by the enemy with ease.
AGR - 1000 Points
The AGR (autonomous ground robot) is air-dropped into the battlefield and it automatically attacks enemies - it can also be remote-controlled at anytime. The AGR is heavily armored and has a powerful machine gun and a rocket launcher.
Stealth Chopper - 1100 Points
An advanced version of the normal Attack Helicopter from previous Call of Duty games. The Stealth Chopper circles around the map and hunts for enemies, but it doesn't appear on the enemy radar.
Orbital VSAT - 1200 Points
Shows enemies on the radar, but also the direction they are facing - it shows the enemies on the radar in real time. The Orbital VSAT can't be shot down.
Escort Drone - 1250 Points
The Escort Drone follows you around and will attempt to kill any enemies you encounter. This scorestreak reward is equivalent to the AH-6 Overwatch pointstreak from MW3, but it's more advanced.
EMP Systems - 1300 Points
Temporarily disables enemy electronics, e.g. enemy players will have no HUD (head-up display) and can't call in Scorestreak rewards.
Warthog - 1400 Points
Calls in an A-10 Thunderbolt that provides air support by doing several strafe runs on the map.
Lodestar - 1500 Points
Equivalent to the Modern Warfare 3 Reaper Pointstreak; it allows you to launch multiple missiles from the Lodestar to the ground.
VTOL Warship - 1600 Points
Allows you to be the gunner of a powerful VTOL Warship. It works exactly like the Chopper Gunner killstreak from Call of Duty: Black Ops.
K9 Unit - 1700 Points
Spawns multiple attack dogs that hunt down enemy players.
Swarm - 1900 Points
Calls in a swarm of Hunter Killer drones that search and destroy enemies on the map.