Sour cream (in north american english, australian english and new zealand english) or soured cream (british english) is a dairy product obtained by fermenting regular cream with certain kinds of lactic acid bacteria. the bacterial culture, which is introduced either deliberately or naturally, sours and thickens the cream. its name comes from the production of lactic acid by bacterial fermentation, which is called souring. crème fraîche is one type of sour cream with a high fat content and less sour taste.

Source: Wikipedia