A Drain prevents water from occupying the tile it is constructed on. They can be used to stop water from flooding certain areas if placed correctly and are often placed on the entrances of certain rooms, such as showers, to prevent them from flooding your corridors. Take notice that Drains do not actually reduce the amount of water, they simply prevent water from occupying a tile. They do not need to be connected to the water system.
Drains can be placed beneath other objects, e.g. a door.