Any house with water pipes -- which is every house -- needs a variety of shutoff valves to stop water flow, or repairs would be impossible, and every leak would quickly become an emergency. In most houses, toilets and faucets have their own valves, while inline valves control sections of the plumbing, and a main valve controls the entire house. Each valve can have one of the three main valve designs common in plumbing.
Gate valves are heavy-duty valves that plumbers often use to control flow in 1-inch-diameter or larger water pipes in residential and industrial systems. When you close a gate valve by turning the handle counterclockwise, a metal gate descends to the bottom of the valve, gradually restricting and finally stopping the water flow. This type of valve is particularly vulnerable to mineral deposits, which can collect on the bottom of the gate or the seat. They prevent the gate from seating completely and cause the valve to leak. You can usually remove them by disassembling the valve.
Stop valves are similar to gate valves, but they have a key difference that makes them more useful for 3/4-inch and smaller pipes than for larger ones. When you screw the handle counterclockwise to close a stop valve, the valve stem presses against the water inlet toward the bottom of the valve. The actual seal is formed by a washer attached to the end of the valve. When the washer wears out, which is a common occurrence, the valve leaks until you replace it. The procedure for replacing it is simple, but you must first find an alternative way to turn off the water.
The small shutoff valves you find under sinks and toilets are often ball valves. The valve chamber contains a close-fitting plastic or metal ball with a hole in it, and it's closed when the hole is aligned perpendicular to the water flow. The operation of a ball-valve differs from either a gate or stop valve because you can only turn the handle through 90 degrees. The valve is open when the lever is parallel to the water flow and closed when it's perpendicular. Ball valves can also be large -- the main shutoff valve for the house is often a ball valve located on the street.
The faucets on sinks, bathtubs and shower are also water shutoff valves, and they can have sophisticated designs. In addition to those that function like stop valves and ball valves, faucets can have internal cartridges. Single-handled faucets are finely tuned to allow them to regulate water temperature by precisely controlling flow from the hot and cold water pipes at the same time. Shower valves often have an internal pressure-regulating mechanism to keep the temperature constant when the pressure in the hot or cold pipes changes. This prevents the water in the shower from becoming scaldingly hot when someone flushes a toilet.