The French version of soured cream. It's twice as rich, and thicker than the British version, made from pasteurised cows' milk to which a lactic bacteria culture has been added. This thickens the cream and gives it a distinctive sharp flavour.
Richer than soured cream, it can be used to lace soups, sauces and stews or it can be spooned over puddings and stirred into sweet dishes. It isn't suitable for whipping though. A half-fat version is available. Plain full-fat yoghurt can be used instead of crème fraîche in cooking as long as you heat it gently - adding a little cornflour can help to prevent it from separating.
The best crème fraîche comes from a strictly controlled area of Normandy called Isigny-sur-Mer and it's sold in larger supermarkets in the UK. It's sold in tubs and has a longer shelf-life than double cream so you can store it in the fridge for a couple of weeks and use it as and when you need it.