The different shut-off valves in a cold water system, and where the water company's pipework ends and yours begins.
Stop valve, gate valve and service valve
A stop valve (stopcock) shuts off the whole incoming supply. It seals against the flow, so it is fitted with its arrow pointing in the direction of flow, and it is the valve used at the mains entry and at the boundary of the property.
A gate valve opens to a full bore with no restriction, so it suits low-pressure pipework such as a gravity feed from a storage cistern. It has no set flow direction and is turned with a wheel; it is not meant for fine flow control.
A service (servicing) valve is a small in-line valve, often turned with a screwdriver, fitted close to one appliance or fitting. It isolates that single item for repair or replacement without draining the rest of the system.
Pillar and mixer taps
A pillar tap is a single tap body that delivers one supply, hot or cold, to its own outlet on a basin or bath. A mixer tap blends the hot and cold supplies together so they leave through one shared spout.
Communication pipe and supply pipe
The underground pipe from the water main to the boundary of the property is the communication pipe. It belongs to the water company, which is responsible for it.
From that boundary onward, the pipe running into the building is the supply pipe, and it belongs to the property owner. The recognised dividing point between the two is the external stop valve, also called the boundary stop tap, usually sited near the boundary. An internal stop valve (the main stopcock) inside the building is a separate valve on the customer's own supply pipe.
Draining the system
A drain-off cock (draining tap) is fitted at the lowest point of the pipework. Water collects there under gravity, so opening it empties that part of the system for a repair or before a hard frost.