Sour cream

From wikipedia:

Sour cream or soured cream is a dairy product rich in fats obtained by fermenting a regular cream by certain kinds of lactic acid bacteria. The bacterial culture, introduced either deliberately or naturally, sours and thickens the cream. Although sour cream is only mildly sour in taste, its name stems from the production of lactic acid by bacterial fermentation, a process referred to as "souring"..

Crème fraiche (French pronunciation: [kʁɛm fʁɛʃ], "fresh cream"; from French crème fraîche) is a soured cream containing about 28% butterfat and with a pH of around 4.5. It is soured with bacterial culture, but is thicker, and less sour than sour cream.

From answers.yahoo.com:

I am a former chef and there is a big difference, sour cream is made with milk, cream and thickeners and gums to keep it together, creme fraiche is just thickened cream with a souring agent, I made it as a chef with just whipping cream and buttermilk, you can use S.C as a substitute for creme fraiche, but the sour cream has to be a full fat, 15% or higher, here in Canada I can buy one that is 30% and C.F is 35-40% BF. Just make sure if your using it in hot dishes not to boil it or it will split, C.F does not, or add a little cornstarch to the S.C and add it in the last 2-4 minutes of light simmering.