Valves are typically used to control either gas or water, either allowing or preventing it from reaching specific areas of your home.
A stop cock will enable you to stop water reaching an entire property. Whilst, a Hose Union Bib Tap allows you to feed water to a hosepipe, normally in a garden.
Gas Valves may be connected if you have a gas-powered appliance, for example a cooker.
Whilst most types of valve require human interaction with a lever or a cog in order to control, some valves are powered by the appliance itself, for example a Float Valve which allows the toilet bowl to empty when it is flushed.