A poppet valve is a valve typically used to control the timing and quantity of petrol (gas) or vapour flow into or out of an engine, but with many other applications.
Overhead camshaft engine
Oscillating poppet valve on one of Chapelon's rebuilt 4-6-2 locomotives.
Image: Poppetvalve
A valve is a device or natural object that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
These water valves are operated by rotary handles.
Valve disc
The shut off butterfly valve for a Francis turbine at Gordon Power Station, Tasmania
Ball valve