The Presta valve is one of two main types of valves used on bicycle tires. (The Schrader valve is the other.)
Presta valves use a small locking nut to hold the valve shut as opposed to a Schraders spring loaded mechanism. This makes the Presta valve slightly more cumbersome to use, but also allows the valve itself to be thinner. Because of its relative size it requires a smaller hole in the rim which allows for stronger or thinner rims. That makes the Presta the valve of choice for professionals and other people that need a strong or thin rims.