A flow controller is a device used to regulate the flow of liquids or gases. The term can refer to a variety of devices, including valves, taps, spigots, faucets, and sluices. Valves are mechanical devices that control the flow of liquids or gases by opening, closing, or partially obstructing passageways. Taps are similar to valves, but they are usually smaller and simpler in design. Spigots, faucets, and sluices are all types of taps that are used to control the flow of liquids.