What is a ranked or social match on Halo 3?

The maps on Halo 3 are:

Regular Map Pack

Construct

Epitaph

Guardian

High Ground

Isolation (not in Matchmaking anymore)

Last Resort

Narrows

Sandtrap

Snowbound

The Pit

Valhalla

Heroic Map Pack

Foundry

Rat's Nest

Standoff

Legendary Map Pack

Avalanche

Blackout

Ghost Town

Mythic Map Pack

Assembly

Citadel

Heretic

Longshore

Orbital

Sandbox

Standalone

Cold Storage

Of these, some have multiple variants used in Matchmaking games such as Infection, Grifball, and others.