Halo 3's first map pack, includes "Standoff", "Rat's Nest", and "Foundry".

Standoff’s symmetrical valley, with its entrenched bases and fields of boulders is ideal for mid-sized objective and Slayer game types.

Rat’s Nest’s vast, labyrinthine passages bring something quite new to the Halo multiplayer experience – an indoor vehicle paradise, strongly influenced by the Campaign mode ideal for big team battles.

Finally, Foundry is the ultimate Forge map – players can edit every single object in this voluminous industrial warehouse, place stairways, walls, bridges and tunnels to create an entirely new play space and build almost any kind of map they can think of.

Source - Xbox.com