In Towns, Buildings can be built and placed to enable advanced features of the game. For instance, the Mineshaft allows endless mining of rare minerals. Farms are also considered buildings and information can be found here.
Buildings, like walls, can be toggled transparent by using either CTRL key.
Buildings can be placed by accessing the Buildings menu from the right-side bar. From there, simply clicking the desired building, then placing it in a legitimate place - some buildings can be placed anywhere above ground, others can only be placed on humus or only underground on dirt, for example. Shift-clicking produces duplicates of the selected building.
Once placed, Townsfolk will bring the required materials to create the building, if they are available. Until finished, the building will have a large red cross over it.
Once finished, the cross is removed and the building is ready for use.
Note: The image above is of a retired building, and will not be seen in the current game version.