Sour cream can be either cultured or acidified, though the former is more common in the production of commercial sour cream.
Sour cream probably first occurred by accident, when cream rose to the surface of fresh milk and naturally present bacterial cultures soured it. Acidified sour cream can be made made by adding an acid such as vinegar or lemon juice to light cream and letting it sit at room temperature. Commercially, sour cream is produced by adding a culture of Streptococcus lactis to light cream and incubating the mixture at room temperature until the desired thickness is reached. The culture produces lactic acid, which coagulates the protein, producing sour cream's characteristic thickness. Nonfat milk solids and stabilizers are often added to commercial sour cream.
The milk fat content of sour cream depends on the the milk or cream from which it's made. Usually sour cream is made from light cream and contains 18-20% MF. "Light" or "lite" sour cream contains about 40% less fat, while non-fat sour cream has no more than .5% MF; thus it must contain stabilizers as a thickening aid.
Store sour cream in the fridge, and if it separates, just stir it up to reconstitute.
Sour cream is used in baking, where its richness and acidity produces a moist and tender texture. It's also used in cooking, though to prevent curdling, it should be added as late as possible in the heating process, and stirred in gradually. Finally, it's a delicious condiment for tacos, soups, and dips. You can use commercial sour cream to make crème fraîche.