In most recipes, heavy cream can be substituted for creme fraiche, but in dishes that require that the cream be thick, or when the tanginess of creme fraiche is important, you will have to either go out and buy a small and very expensive little container or make it yourself.
Translated literally, creme fraiche just means fresh cream, but creme fraiche is actually more like what we call sour cream than it is like heavy whipping cream. The tang makes a delightful counterbalance to the sugar in sweet dishes. On a traditional French dairy farm, creme fraich is made by letting fresh cream sit until the natural beneficial bacteria in the cream ferment the milk sugar, converting it to lactic acid, which causes the cream to thicken. These days, most creme fraiche is made by inoculating pasteurized cream, in which the naturally occurring bacteria have been killed, with cloned bacteria, all genetically identical, grown in a laboratory. You can produce the same effect at home by combining heavy cream with a small amount of buttermilk, which contains the active culture. Use 2 cups of cream to 1/4 of buttermilk and let the mixture sit, covered, in a warm place for 24 hours.
Creme fraiche keeps longer than heavy cream because it has already been allowed to sour, but with good rather than harmful bacteria. Don't try to substitute sour cream for creme fraiche in sauces that are served hot, because sour cream has lower butterfat content and will curdle when heated. Creme fraiche will keep for several weeks in the refrigerator.