A valve stem is usually a piece of cylindrical metal that protrudes from an inner tube and through a wheel to allow a user to inflate the tire or tube. The valve stem may or may not be attached to an inner tube; it may instead be a standalone unit that allows air to pass directly into a tire and wheel set up that uses no tube. In many cases, the valve stem will close by itself in many cases to prevent air from leaking out once the inflation process is completed, though some stems must be sealed off manually.
The most common type of valve stem is the Schrader valve, which is common on cars, motorcycles, and even bicycles. It is sometimes known simply as a car tire valve because it is most recognizable as being used on car tires. The valve stem is usually threaded on the tip to allow a cap to be screwed onto it; this cap prevents dirt and grit from getting inside the valve, thereby blocking the passage of air or triggering a release of air. The core of the valve stem is usually removable by unthreading it from the stem cylinder, though removal of the core is not usually necessary. The core prevents air from escaping the valve stem, and when depressed, it opens the chamber to allow air to pass into the tire or tube during the inflation process.
Another common type of valve stem is the Presta valve, which is most commonly used on bicycles. This type of valve is much narrower than a Schrader valve, and the core is a different design than that of a Schrader valve as well. The core is positioned within the stem cylinder, but it is pressed open or pulled closed using a threaded barrel that extends beyond the tip of the valve cylinder. When unscrewed, the barrel allows air to pass through the cylinder, and when screwed tightly, it seals off the chamber. These valves are not common in other applications beyond bicycle tire and tube setups.
Tires and tubes are not the only application for valve stems. They can be used on just about any pressurized mechanism; some automobile shock absorbers as well as motorcycle and bicycle shock absorbers may feature these valves to allow for customization of the shock's responsiveness. Air conditioning units sometimes feature these valves, as do some mechanisms related to plumbing applications.