Fructose is the chemical name for the kind of sugar found in most fruits. Sugar found in corn and human blood is called glucose. Normal table sugar, produced from sugar cane or sugar beets, is called sucrose.
Fructose is sweeter than glucose and sucrose. It is thus useful for people who wish to reduce the sugar in their diet without reducing sweetness. It's also useful for people who simply want extra-sweet food. Note that reducing the sugar in a recipe can have bad effects; sugar is used for browning and for mold prevention.