Technically, ricotta is not a cheese at all, but a cheese by-product. Its name, ricotta, means cooked again, an obvious reference to the production method used to make it.
Ricotta is made from the whey drained from such cheeses as mozzarella, provolone, and other cheeses. American ricotta is generally made with a combination of whey and whole, low-fat, or skim cow's milk.
Unrelated to soft ricotta, ricotta salada is made of sheep's milk. The liquid is pressed out and the solids are compacted into rounds, enabling it's crumbly, firm texture to be cut with a knife.
Ricotta cheese is highly perishable and should be snowy white in color, never yellow. Check containers for an expiration date. Ricotta should always be rerigerated in its container with the lid tightly affixed. Once opened, use within one week, discard if there are any signs of mold.
Ricotta can be frozen for up to six months & thawed in the refrigerator.
Tofu or dry cottage cheese may be substituted for ricotta in equal measures, but the flavor will be milder and a bit saltier.