Killstreak

A killstreak is when a player gets two or more kills in a row. In Call of Duty 4: Modern Warfare, Call of Duty: World at War, and Call of Duty: Modern Warfare 2 the player is given various rewards depending on the length of the killstreak.

Call of Duty 4: Modern Warfare

 * UAV Recon - 4 kills
 * Airstrike - 5 kills
 * Helicopter Support - 7 kills

Call of Duty: World at War

 * Recon Plane - 3 kills
 * Artillery Strike - 5 kills
 * Dogs - 7 kills

Call of Duty: Modern Warfare 2
In this game, players can choose the three killstreak rewards they can earn, rather than just the standard three. All rewards require 1 less kill if the user is utilising the perk Hardline.


 * UAV Recon- 3 kills
 * Counter-UAV- 4 kills
 * Care Package - 4 kills
 * Predator Missile - 5 kills
 * Sentry Gun - 5 Kills
 * Precision Airstrike - 6 kills
 * Harrier Airstrike - 7 kills
 * Attack Helicopter - 7 kills
 * Emergency Airdrop - 8 kills
 * Stealth Bomber- 9 kills
 * Pave Low - 9 kills
 * Chopper Gunner - 11 kills
 * AC-130 - 11 kills
 * EMP- 15 kills
 * Tactical Nuke - 25 kills

Death streaks
In Call of Duty: Modern Warfare 2, death streaks were introduced, where players would be rewarded for multiple deaths without a kill, to give players having a bad game a small advantage. Only one death streak reward can be chosen.

There apparently was a perk originally planned named "The Dead Man's Hand". I can be seen in the game Guide, which is pre-launch. Nothing is known about what it does.
 * Painkiller - 3 deaths
 * Copycat - 4 deaths
 * Final Stand - 4 deaths
 * Martyrdom - 4 deaths

Trivia

 * In Call of Duty 4 : Modern Warfare, and Call of Duty : World At War, killstreaks DO NOT stack. While in Call of Duty : Modern Warfare 2 they do.