Start with a pint (or more, if desired) of heavy cream in a glass or metal bowl.
Then, stir in a tablespoon or two of buttermilk. Usually the 1% variety will do just fine.
THEN ... cover it with a cloth and set it on top of your refrigerator (that's right - ON TOP) or on your countertop overnight. (that's right - 8 hours or more)
The result will be what restaurants call "Crème Fraîche" which is french for (oddly enough) "fresh cream", instead it is a cream that has soured overnight due to the enzymes present in the buttermilk and is by no means "fresh", BUT very tasty indeed. After the overnight jaunt with the enzymes, it will stay edible for at least at week in the fridge. If you can find a lower fat content heavy cream, bully for you. And in fact, you can probably buy sour cream cheaper than you can heavy cream any day, so why do I bother with this little ditty? To show you that a restaurant that touts "Crème Fraîche" may very well be serving sour cream instead, and only wanting to sound more expensive than they are.
In conclusion, "Crème Fraîche" can be used synonymously with sour cream in any restaurant with no fear of retribution, because thay are both "creams that have been soured" by chemical means.