The Spectre is an assault vehicle, designed by the Covenant to be a hybrid assault and reconnaissance vehicle.It appears multiple times throughout the campaign of Halo 2, mainly in the levels where you play as the Arbiter. The vehicle is a clever mix of a Ghost and a Warthog. Many people believe that this is what it was always supposed to be: a Covenant Warthog. It had the same general speed as a Warthog, the same style handling, even though it hovered instead of being bound by the laws of physics, like the Warthog. It had a slightly larger capacity for carrying passengers than the Warthog. The Hog could carry a driver, a passenger and a gunner. One human or Covenant occupied the driver's seat in the Spectre, one on either wing of the vehicle, and one in the turret at the back.
The extra passenger would've made the vehicle a plausible threat in multiplayer, allowing an entire team to bail into a base at once. Unfortunately it only ever appeared on Waterworks, in larger team battles, thus diluting the effect when there are 8 players on a team anyway.
The turret was a unique weapon in Halo 2, it was nothing like any other mounted or personal weapon in the game. It was best at close to medium range. The turret fired a high frequency energy beam. Fully automatic, it was could be compared in power to it's counterpart human turret in the Warthog. Good at close to medium range, but the accuracy broke up terribly when you tried to take on a distant target with it.
It is ONLY in Halo 2, although originally designed for Halo 1.