What is horde on gears of war 2

Gears of war 2 horde is a game type that pits you against 50 waves of all types of the unending locust.
This game type can be play cooprativley or solo, the 4 difficulties are present in horde, casual, normal, hardcore and insane. Basically you start at round one and try to survive to round 50. These rounds get harder and harder as the game progresses and resets every 10 levels. (Wave 1, you are against wretches and drones, wave 10 you are against all locust types including bloodmounts, then wave 11 will be drones and wretches only again, but stronger than wave 10).
You can only play as the gears and not locust, as that would be stupid, but you can play on all maps, and co-operation with your time is needed!
On the XBox 360 version of the game, completing all 50 waves of horde unlocks the 'Hoard the Horde' Achievement.