Bicycle inner tubes have two major types of valves: Presta and Schrader. A third, rarer type of valve is the Woods or Dunlop style. Presta, Schrader and Woods valves are each most common on particular types of bikes, but they function largely the same and are frequently, but not always, interchangeable on wheel rims.
Schrader valves are the same type of valve used on automobile tires. The depressible part of the valve, a small pin, sits inside the valve stem. A spring-loaded removable core inside the valve stem keeps air from escaping when the depressible pin is not lowered. A plastic cap comes with each inner tube to protect the valve from road debris and water. Schrader valves are more common on mountain, comfort, hybrid and child bike inner tubes.
Presta valves are longer, skinnier and lighter than Schrader valves. They feature a metal screw-on top attached to the depressible valve pin, which protects the valve from leaking air. The Presta valve, with no internal spring, relies on the external air pressure being lower than the internal pressure to keep the valve closed. A new Presta valve inner tube comes with a cap similar to a Schrader cap. Bike mechanic Sheldon Brown notes that this cap's only purpose is to protect the rolled-up inner tube from puncture by the valve. A ring-shaped nut screws onto the body of the Presta valve and holds it firmly in place against the rim for easy inflating. Prestas are more common on road bike inner tubes, including sew-up or tubular models.
Woods or Dunlop
Rarely seen in the United States, the Woods or Dunlop valve has a base similar in size to the Schrader valve and a narrower piece at the top sized similarly to a Presta valve. The valve core itself is like a Schrader core. A ring holds the valve in place against the wheel rim, just as in the Presta valve, except the Dunlop valve ring is airtight. The valve is replaceable and can be moved from inner tube to inner tube.
You can buy Presta and Schrader valve adapters for use with pumps that only accommodate the opposite type of valve. Note that Schrader and Woods valves will not fit into wheel rims designed for Presta valves without enlarging the valve hole.