What is 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: [k??m f???], "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.

I found definitions of sour cream and creme fraiche from the book The Chef's Companion: A Culinary Dictionary by Elizabeth Reilly.

Sour Cream: cream commercially fermented with a lactic culture and usually 18 to 20 percent fat

Creme Fraiche: French for heavy cream with a lactic culture introduced; the culture acts as a preservative and gives a characteristic tangy flavor"

The Cook's Thesaurus recommends substituting "equal parts sour cream and heavy cream" and warns that just sour cream alone "has a lower fat content, and so it's more likely to curdle if boiled with an acidic ingredient." You can read the entire entry here.

I also found an online source that you may find interesting. This is a chart from the USDA National Agricultural Library that shows exactly what nutrients are found in sour cream.

I thought crème fraîche was traditionally made by letting unpasteurised double (heavy) cream sour naturally, so there was traditionally, at least, no milk or thickeners in crème fraîche. I'm pretty sure that French crème fraîche is made that way to this day. Sour cream was traditionally made in a similar way - but these days the cream is pasteurised first, and the bacterial cultures re-introduced.

Crème fraîche is not so sour, or so thick, as sour cream, and it has a higher fat content (about 28% compared to 12-16% for sour cream) which means it can take a higher heat - so it doesn't split as easily as sour cream in hot dishes.

Crema Mexicana is similar to crème fraîche and can be used in hot dishes.

Crème Fraiche vs. Sour Cream

Crème fraîche and sour cream are not the same product; however, they are very similar in their rich, tangy flavours. In recipes where they are not the main ingredient, they can easily be substituted for one another.