The Spectre is very similar to the Prowler. The differences being that the Spectre can boost,the driver is more protected, and the gunner is more exposed. Used mainly by the Elites and when the Elites left the Covenant, the Prowler quickly replaced it in the Covenant Loyalist Army.
Spectres saw very little use in Halo 2's campaign, and were even rarer in multiplayer. The Spectre was not present in Halo 3, with it's role being filled by the Brute Prowler.