Structures are small auto-generated areas of the map that appear to have been made and abandoned by some unknown person or race. Structures generally contain rare materials, and may also have Chests with Food, Minerals, or high level Tools for players to loot.
Dungeons are underground rooms made of Mossy Cobblestone, often containing chests with loot.
Camps are above ground areas of made up of tents and crates supposedly left behind by long-lost explorers. Camps contain low to medium level loot to aid in survival while exploring the map.