The Schrader valve is the main type of air valve used on tires. (The Presta valve is also fairly common in bicycles.)

The Schrader valve uses a spring mechanism to hold the valve shut. Pressing down on the mechanism opens an air gap and allows air to travel through the valve. When the mechanism is released the spring (and air pressure) close the gap. It's a very simple, effective and cheap valve which is why it is so common.