The most important difference you should know between yogurt and crème fraîche is that yogurt is made from milk while crème fraîche is made from heavy cream; and this initial difference of starting ingredients affects the culturing process and ultimately the flavor and texture of the final product.
Mascarpone is the Italian version of creme fraiche, still soured by a lactic culture but is milder and sweeter. They all have a delightful, piquant edge and are perfect for partnering dense chocolate cakes.