In France, creme fraiche is a naturally thickened cream made with unpasteurized cream and provides a tart, buttery flavor. In the U.S. the product is made with pasteurized cream and fermented with either buttermilk or sour cream. The resulting product is not quite the same as the French version but still very flavorful. You can purchase commercially prepare creme fraiche or make your own which is quite simple, but since you are on this page it is just a click away.
Creme fraiche is used as a base for sauces, or like cheese as part of an appetizer (It pairs fantastically with Caviar on Toast Points or Crackers). It is also used in desserts.