A bunker is a type of underground structure that is generated automatically by the game whenever a new temperate world is created. Bunkers are typically small, relatively simple buildings, and usually contain lootable containers like red chests, industrial chests (commonly known as ecological chests or eco crates), barrels, sacks, and the like, along with various items and decorations left by the former occupants. Unlike dungeons, bunkers have no red enemy protectors or guardian brains, although they often have turrets and spikes. On occasion, a bunker will be covered by a broken protector.
Compared to dungeons, bunkers are relatively easy to raid, and so are often the main source of lootable goodies for newer players.
Players have come up with some informal ways to designate certain specialized bunkers:
"Luxury Bunker" (often shortened to "lux bunker" or just "lux") - This is a multi-story rectangular structure, usually full of bookcases, and made primarily of wood products with a surrounding layer of reinforced brass. At least one or two floors usually have turrets, and there's almost always a red chest in the basement room, unless earth has fallen into the room during the generation process and obliterated the chest. It may also have a mounted head on the middle floor, or a painting, globe, airship in a bottle, or solar system diorama on the top floor -- or, if you're really lucky, both. Since this is one of only two ways to find the most coveted rares other than onyx, people tend to announce in chat when they come across them. Note that there's only a small chance that a painting or mounted head will generate in this type of bunker to begin with, and an even smaller chance that such a painting will be anything other than an Une Pipe or the mounted head anything other than a deer head.
"Head Bunker" - This is a loose term to denote any bunker that has a chance of generating with a mounted head. Head bunkers take one of only a few different shapes, and generate in all world types. They are usually protected only by a few turrets, and some of them also generate with a red chest. Note that certain mounted heads can only be found in bunkers of certain biomes. For more information about this, go to the page for the individual mounted head that you're interested in. Also, as with luxury bunkers, not every head bunker will have a mounted head. There is one head bunker, however, that does always generate with a mounted head. This bunker has a long main floor with the head on one side and a storeroom of gunpowder or pitch on the other, and a small basement room that always holds a red chest, three skeletons, and a brazier.
"Ice Sculpture Bunker" - Found only in arctic biomes, ice sculpture bunkers are the only places to find, you guessed it, ice sculptures. There are several variations of ice sculpture bunkers, but they are all marble-themed, with marble backdrop and a few marble pedestals or columns inside. In all variations, there's also a broken protector that the player must mine, as well as a turret or two. Unlike head bunkers and luxury bunkers, these bunkers always generate with one or more ice sculptures inside.
"Gift Bunker" - Gift bunkers are found only in holiday arctic worlds. They're usually small, square, cork-lined rooms that contain gift boxes.
"Stocking Bunker" - Also found only in holiday arctic worlds, the stocking bunker is a small, metal-themed structure that houses an onyx stocking.