Ricotta—which translates from Italian as "re-cooked"—is a soft, fresh cheese made from the whey leftover in the production of other cheeses. In the traditional method, the whey is left to ferment for up to 24 hours, then heated until the protein in the whey forms into small curds. These days, ricotta is often made with a combination of whey and fresh milk. The texture is creamy, though slightly grainy, and the flavor is milky and sweet.
Ricotta is a common ingredient in Italian desserts, as well as filled and baked pasta dishes.
Ricotta salata is ricotta that's been salted, pressed, and aged for at least three months, creating a much firmer, drier cheese. It's ideal for grating over pasta or salads, but should never be substituted for plain ricotta (or vice-versa), as the two are quite different.
In some dishes, cottage cheese may be substituted, though the curds are generally larger. To achieve a texture closer to ricotta, whir the cottage cheese in a blender or food processor for a few minutes.
Ricotta is usually available in whole-milk and part-skim versions. For a special treat, look for fresh ricotta sold in Italian groceries; though it's highly perishable, these smaller-batch ricottas have a much more complex flavor than the mass-produced versions.
Keep fresh ricotta well-wrapped in the refrigerator and use within two days (even after one day, you'll notice a decline in quality). Supermarket ricotta will last several days longer in the fridge once it's opened.