Definition of valve stem oil seals?

The valves are in the head of the engine. They look like a Golf tee. They open and close to let air in and the exhaust out. The oil seal is a small cup that the stem of the valve goes through to keep oil from running down the stem and into the cylinder. It is shaped like an umbrella or upside down coffee cup. They wear out over time or get hard and then the car will start to smoke. Usually when it is first started. The oil runs down the stem when it is shut off and burns when the engine is first started. There is usually not enough oil for it to show while driving.