Cream that has soured by the action of lactic-acid bacteria, sour cream is thick and has a slight tang. Often used as a garnish for soups and stews, it's also used in baking in (think sour cream coffee cakes and cheesecake), in puréed soups, and as a base for dips.
When used as a garnish, in soups, or the base for a dressing or dip, crème fraîche (the French version), which is thicker and less sour, crema (Mexican sour cream), or plain yogurt (thicker, richer Greek yogurt especially) can often substitute. If substituting for sour cream in baking, however, other adjustments may need to be made.
To keep sour cream from curdling when heated over high heat, stir in a bit of flour.