A valve is a mechanism that opens and closes to control the flow of fluids. In a scientific context, fluids include both liquids and gases – any substance that can flow freely – and valves may use either. Valves control fluid flow in motors, plumbing, irrigation, pneumatic and hydraulic systems. Sometimes the venerable pre-transistor vacuum tube is referred to as a valve as well.
The simplest valves are one way-flap mechanisms that allow flow in one direction and shut with back-flow pressure to stop flow in the direction of origin. (This is essentially how the valves in a heart work.) In addition to running automatically through the physics of flow itself, valves can also work through pressure or temperature and may be run by a motor or operated manually.
Manual valves are usually operated by levers, pedals, hand knobs or wheels to crank a screw-turned stem. A valve sometimes has geared mechanical insides that actuate the valve ball or disc against its seat, which may include a gasket. A valve may control fluid flow to one or, often, as many as four ports. Valves vary greatly in cost and size from as small as 0.1mm (millimeter) to as big as 5m (meters) in special applications.
Valve also happens to be the name of the game developer and purveyor of the largest game distribution service: Steam.
Watch a brief demonstration of how ball valves work: