Most compression valves have a threaded metal spindle with a disc-shaped stem washer on the end. When the spindle is screwed all the way in, the stem washer covers a hole, and water flow to the spout is stopped. Drips from the spout happen when the seal between the stem washer and the rim of the hole (called the “valve seat”) is imperfect. Usually, replacing the stem washer is enough to stop the drip.
There are many ways to classify sink faucets, but perhaps the most useful distinction is compression style versus washerless. These names refer to the type of mechanism inside the faucet. Many older faucets are compression type, but most newer ones are washerless, which can be one of three principal types: cartridge, ball, or disc. All one-handle faucets are washerless. The best way to tell if your faucet is a compression type or washerless is to turn the handle. With compression faucets, you can feel the compression building as you crank the handle, even after the water flow has stopped. On washerless models, the handle comes to an abrupt stop.