A gate valve is an older style of shutoff valve that raises and lowers a solid wedge, or gate, to open and close a water line. You turn a round handle several full turns to work it. Gate valves seal well when new but tend to seize or leak with age, so plumbers now favor ball valves for shutoffs.
A gate valve is one of the oldest and most common shutoff valves in plumbing, especially in homes built before the switch to ball valves. Its name describes exactly how it works. Inside the valve body is a solid metal wedge, the gate, that slides down to block the pipe or lifts up out of the way to let water through. Turning the handle raises or lowers that gate.
The handle is a round wheel, and this is the first big difference from a modern ball valve. A gate valve is a multi-turn valve. You have to spin the handle several full turns to move the gate from fully open to fully closed. There is no quick quarter turn and no clear at-a-glance signal of whether it is open or shut. You turn until it stops. When new and clean, a gate valve seals tightly and is well suited to being left fully open or fully closed for long stretches, which is how most shutoffs are used.
The problem is age. Gate valves have more moving parts exposed to water than a ball valve does, and those parts corrode. The threaded stem that raises the gate can seize, and the gate itself can build up mineral scale or rust. The result is the classic failure: a gate valve that has sat untouched for 15 years often will not turn when you finally need it, or turns but no longer seals, leaving a trickle you cannot stop. For example, a homeowner reaching for the old main shutoff during a leak may find the handle frozen solid or spinning uselessly. This is exactly why plumbers now install a ball valve in place of a gate valve on shutoffs during most repairs.
If your home still has gate valves on the main or on appliance shutoffs, it is worth knowing they exist and testing them gently before an emergency. A gate valve that is hard to turn or drips at the stem is near the end of its life, and replacing it with a quarter-turn ball valve is a common, worthwhile upgrade that keeps a required shutoff working.