The Buzzard and Buzzard Attack Chopper are light helicopters.
The Ballad of Gay Tony only knows the Buzzard: an attack chopper armed with rockets and two high caliber machine guns. Grand Theft Auto V introduces an unarmed variant of the Buzzard. Presumably to distinguish the two, the armed version is renamed to Buzzard Attack Chopper, whilst the unarmed version is given the old name: simply Buzzard.
Yusuf Amir lets Luis Lopez steal a Buzzard in the mission Sexy Time, that he paints golden. This now golden Buzzard is used by Luis later in the storyline in the mission Not So Fast, and is used by Yusuf himself in the final mission Departure Time, by blowing up Luis' pursuers.
After completing the storyline Yusuf gives Luis this same Buzzard as a gift. It will then start spawning on the Westminster helipad, replacing the Swift.