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, see recipe this page.
Creme fraiche is used as a base for sauces, or like a cheese as part of an appetizer (as shown in the image, table water crackers, topped with creme fraiche and caviar). It is also used in desserts.
If you have time you can make your own, recipe below. You can purchase Creme Fraiche Starter Culture to use rather than buttermilk for making your own. Otherwise you can typically find creme fraiche in well-stocked grocery stores, Trader Joe's, Whole Foods Market or online at Amazon.com: Creme Fraiche
This is easy to make but should be made 1 day ahead so it has time to thicken.
1 cup whipping cream
2 tablespoons buttermilk
Place 1 cup whipping cream in a small saucepan on low and heat just until it is barely warmed. Do not overheat or your won't thicken. Add the warmed cream plus 2 tablespoons buttermilk in a glass container.
Cover and let stand at room temperature (about 70°F) from 8 to 24 hours, or until very thick. Stir well before covering and refrigerate up to 10 days.