What is Cheese Anyway?
Cheese is a rather general term describing curdled milk (or cream). To curdle milk means to separate the whey from the curds. Milk proteins (casein) are ostensibly broken in half. One half precipitates out of the milk, becoming a solid (the curds). The other half remains liquid (the whey), though it ceases to be white. The distinction between true cheese and things like cream cheese, sour cream, mascarpone, creme fraiche, etc. is the way in which the milk is curdled. Milk can be curdled either by acid and/or by rennet. To be considered a true cheese (e.g. cheddar, swiss, brie, et. al.), acid AND rennet coagulation (i.e. curdling) is required.
First, a culture is added to the milk (or is already present in the milk in the case of some raw milk cheeses). This culture "ferments" the milk, slowly lowering the pH (raising the acidity) of the milk. Then, rennet is added - this is an enzyme derived from the fourth stomach chamber of an unweaned ruminant animal (e.g. a calf, kid, or lamb). This enzyme literally breaks apart the proteins in the milk, an action facilitated by the presence of acid (and heat), and separates the milk into curds and whey. The whey is poured off (either to make ricotta, or to feed to hogs). The curds are then cut, releasing more whey, drained, and molded (this is a gross simplification - most of the textural variations in cheese stem from this process). At this point, the curds have become cheese.
False cheeses like creme fraiche, mascarpone, sour cream, etc. are curdled very lightly and without the use of rennet. The acid required can be produced by natural or added bacteria, vinegar, lemon juice, etc. The variations in the acid-curdled "cheeses" come from the extent of acidification, coagulation, variations in fat content, types of cultures, etc. For example, creme fraiche is cream (thus, high in fat) that is lightly coagulated (lower acidity). Sour cream is cream that is more heavily coagulated. This slight difference in acidity and cultures will cause sour cream to fully curdle (i.e. turn chunky) when boiled, whereas creme fraiche will not.
The diversity and complexity of cheesemaking processes is overwhelming. We hope this helps to illustrate (if not simplify) the breadth of the cheese world.
See also our description of the major cheese types.
For more information we highly recommend Cheese: A Connoisseur's Guide to the World's Best by Max McCalman or any of the selections of our fine books.