The Killstreak system has been updated for Call of Duty: Black Ops II. Killstreaks are now Scorestreaks. The basics are still the same. However, the following updates have been made:
Performing almost any action that is beneficial to your team builds your score toward your Scorestreaks. Playing the objective in Objective-based game modes rewards far more points than simply hunting alone for kills.
Dying resets your Scorestreak progress completely. However, you still earn Scorestreak points generated by active streaks, even after death.
Scorestreaks that are unused when you die remain active when you respawn.
Scorestreaks that are unused at the midpoint of multi-round matches—such as Domination—carry over into the next round.
Scorestreak points earned while controlling a piece of machinery (the RC-XD, Sentry Gun, AGR, Hellfire Missile, Dragonfire, Lodestar, or VTOL Warship) are lost if you are killed while in control. You still earn the points toward your overall match score.
Streak rewards can be looped. If you earn your highest streak, you reset to your first reward. However, streak rewards cannot be stacked, so use your early rewards before you earn them again or lose them!