A valve is a device that regulates the flow of materials (gases, fluidized solids, slurries, or liquids) by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but usually are discussed separately such as Proportional valves,Pressure control valves.

Valves are used in a variety of applications including industrial, military, commercial, residential, and transportation.

Oil and gas, power generation, mining, water reticulation, sewerage and chemical manufacturing would constitute the majority of valves used by industry.

Plumbing valves are the most obvious in everyday life, taps for hot and cold water, gas control valves on cookers and barbecues, small valves fitted to washing machines and dishwashers, and safety devices fitted to hot water systems are all simple valves.

Most valves are operated manually, either by a hand wheel or lever. Some valves are 'self-operating' driven by changes in pressure, temperature or flow. These changes in pressure or temperature often act against springs or capsules filled with expanding substances. Examples of this type of valve found commonly are safety valves fitted to hot water systems or steam boilers.
More complex control systems using valves requiring automatic control based on an external input (i.e., regulating flow through a pipe to a changing set point) require an actuator. An actuator will stroke the valve depending on its input and set-up, allowing the valve to be positioned accurately, and allowing fine control over a variety of requirements.
Directional Valve