Killstreak

A Killstreak is when a player gets two or more kills in a row without dying. 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

 * 3 - UAV Recon: Deploys a UAV to scan the map for 30 seconds and reveal enemy locations (enemies with the second tier perk UAV Jammer will not be scanned by the UAV).
 * 5 - Airstrike: Initiates an airstrike on a targeted area.
 * 7 - Attack Helicopter: Deploys either an Mi-28 hind or an AH-1 Cobra, depending on what team you are on, to attack enemy players for 1 minute.

Call of Duty: World at War

 * Recon Plane - 3 kills: Same effect as UAV Recon.
 * Artillery Strike - 5 kills: Calls in artillery shells to strike a designated location.
 * Dogs - 7 kills: Releases several vicious dogs for one minute that attack enemy players.

Call of Duty: Modern Warfare 2
In this game, players can choose up to three killstreak rewards they can earn, rather than just the standard three. 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 (all killstreak rewards will be unlocked at lvl 69). When a killstreak of 8 or more is activated, a message will notify all players in the match, showing the callsign and the streak being used (except in Hardcore or when the killstreak is a Stealth Bomber being deployed by the opposing team). It is worth noting that two killstreaks that share the same killstreak amount cannot be used together, one must be picked over the other. For example, for the 7 kills slot the player can use either the Harrier Strike or the Attack Helicopter, but not both.

In non-Hardcore game modes, the calling of a killstreak of 8 or more (Emergency Airdrop and up) will be announced with the player's callsign in the top-right corner, with the name of the killstreak directly below it (except the Stealth Bomber is not announced to the enemy team). Also, even though a Tactical Nuke is announced to both teams, the player's callsign on the top-right corner will only be shown to players on that player's team.

For controllable killstreaks (Predator Missile, Chopper Gunner and AC-130), if the player is killed while operating the killstreak, he can continue using the killstreak until it is done and then respawn or enter spectator mode as normal. This outcome has been protested by some who feel the killstreak should end if the player dies. Note: The number of kills required to obtain a particular killstreak reward can be reduced by one when using the Hardline Perk.

Deathstreak
Deathstreaks are a feature introduced in Modern Warfare 2. They are designed to boost low morale in the event a player has died a number of times (only by enemy players) without scoring a kill, although their effectiveness is limited in order to discourage players from intentionally dying in streaks to activate them. They will activate upon each respawn the player has after the necessary number of deaths until a kill is scored. When the deathstreak is snapped the deathstreak perk remains in effect for the remainder of that life; the following respawn will not have the deathstreak active.

Note: The number of deaths required for a particular deathstreak can be reduced by one when using the Hardline Pro Perk (except Painkiller).

Trivia

 * In Call of Duty 4: Modern Warfare, and Call of Duty: World At War, killstreaks do not stack, meaning that once a killstreak reward is available it replaces any unused killstreak reward. In Call of Duty: Modern Warfare 2 they stack with the most recent rewards earned being first in the queue.
 * Killstreaks in Call of Duty: Modern Warfare 2 that do count towards other killstreaks will only do so during the same life. For example, a Predator Missile can contribute towards any other killstreak the player has selected of 6 or higher, but if used after the player dies, it will not count towards any killstreaks. By definition, a killstreak cannot contribute towards a lower one. Killstreaks from care package and emergency airdrop don't count towards your current streak.
 * Suicides, deaths by teammates in hardcore or penalty deaths (from ricochet damage or teamkilling) do not count towards deathstreaks. However, these deaths will end a killstreak (even though the player wasn't killed by an enemy).
 * In Modern Warfare 2, the UAV, Counter UAV and EMP are the only killstreaks that do not directly kill enemy players.
 * It is really fun to killstreak boost with your friends. You hide in the course on opposite teams and they use tactical insertion. You continuously kill them until you get your killstreaks. It's really fun.
 * Painkiller (Juggernaut) and Martyrdom return in Modern Warfare 2 as deathstreaks, not perks.
 * Treyarch has confirmed that killstreaks will no longer build to other killstreaks in Call of Duty: Black Ops.
 * They have also announced that Black Ops will not have a game-ending killstreak like the Tactical Nuke.
 * Care Packages in MW2 do not affect the players killstreak. Sentry Gun airdrop does affect it though.