Shelter
A Shelter is basically a Blockhead's home, a place that it is safe from the elements.
Appearance
Shelters can be simple as a little underground bunker meeting basic needs or as complicated as a five-story golden palace with diamond chandeliers.
Features
A shelter needs light in it and a barrier of solid blocks overhead to count as a shelter or the inhabiting Blockhead's environment meter will go down due to the lack of light or occurrence of rain. A source of heat (campfire or other crafting surface that requires fuel) may also be needed. Doors or other barriers will keep dropbears from attacking at night.
Shelters normally have chests in them to store goods and sometimes the chests will be organized depending on what type of shelter it is. Some type of bed for more efficient sleeping is also useful.
While the various crafting surfaces do not have to be kept inside a shelter, it's typical for them to be located there.
When it comes to what is needed to have a shelter would be near the top.