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The narrow valve used on most high-performance bicycles, and all tubulars. Also known as a "French valve". The wider Schrader (automotive style) valve is used on children's bicycles and utility bicycles.
Presta valves have built-in valve caps, which must be opened before you can pump them up. These caps are "captive" nuts, which cannot be removed. Since there is no spring in a Presta valve, this knurled nut must be retightened after inflating the tire, or the valve may leak slowly.
Presta valves are light, and don't require as large a hole in the rim. Since the don't use a spring, they can be easier to pump with a hand pump.
Separate valve caps are not needed with Presta valves. The caps that come with Presta tubes and tubulars are there so that the pointy end of the valve won't puncture your spare tube or tubular while it is rolled up. There's no reason to use them in actual riding