A ball valve is a shutoff valve that uses a rotating ball with a hole through it to start or stop water flow. A quarter turn of the handle, 90 degrees, goes from fully open to fully closed. Ball valves are reliable, quick to operate, and less likely to seize or leak than older washer-style valves.
A ball valve is one of the most common shutoff valves in modern plumbing, and its design is easy to picture. Inside the valve body is a metal ball with a hole bored straight through it. When the hole lines up with the pipe, water flows through freely. When the ball is rotated so the hole points across the pipe, the solid side of the ball blocks the opening and flow stops completely.
The handle is what turns that ball, and it moves only a quarter turn. Rotate the lever 90 degrees and the valve goes from fully open to fully closed, with no in-between guessing. When the handle sits in line with the pipe, the valve is open. When the handle sits across the pipe, it is closed. That makes a ball valve fast to operate and easy to read at a glance, which is why plumbers reach for it on main shutoffs and supply lines.
The bigger reason ball valves have taken over is reliability. Older shutoff valves, called gate valves or multi-turn valves, use a rubber washer and a threaded stem that you crank several times. Those parts corrode, stiffen, and stick, so a valve that has sat untouched for years often will not close, or will not seal, exactly when you finally need it in an emergency. A ball valve has fewer parts to fail. The ball turns against a smooth seat, so it rarely sticks and rarely leaks, even after years of sitting in one position.
You will find ball valves throughout a home. A common example is the main water shutoff, where a lever-handle ball valve lets you cut water to the whole house with one quick turn during a burst pipe or a repair. They also serve individual appliances, branch lines, and irrigation. Because a quarter turn either fully opens or fully closes them, ball valves are best as on-off shutoffs rather than for throttling flow partway, which can wear the seat over time. For dialing in flow, a globe valve is the better tool.