Ricotta means recooked. It is made by heating whey left from another cheese until the remaining proteins set into a fine, moist curd.
Fresh ricotta is mildly sweet, with no rind and no aging. In the tub most shops sell, the whey comes from cow's-milk cheesemaking. In Italy, a fine ricotta is often the whey of a sheep's-milk pecorino. Ricotta salata is the salted, dried form, used for grating.
History
Whey cheese is a cheese of thrift. After a dairy made a rennet cheese, the whey still held proteins that would set if the liquid was heated again. That second cooking is the ricotta. The practice is old anywhere people made cheese, and in Italy it became its own ingredient, not a leftover to hide.
Different wheys make different ricottas. Sheep whey, left from pecorino in Lazio, Sardinia, and the south, gives a rich fresh cheese for pasta fillings and for sweets such as cannoli and cassata. Cow's-milk whey ricotta became the supermarket tub in the United States and in much of Italy as well. Both are ricotta. A recipe from a Sicilian pastry shop and a recipe from an American lasagna pan may not mean the same richness.
How it is made
Whey is heated until fine curds rise. Those curds are skimmed and drained. Acid may be added to help the set. The cheese is not pressed into a wheel and not ripened. How long it drains decides whether it is loose and milky or firm enough to hold in a filling. Whole milk is sometimes added back to whey, or even used as the main liquid, in industrial ricotta. That product can be very pleasant. It is a step away from whey-only ricotta.
Ricotta salata takes the fresh cheese, salts it, and presses and dries it until it can be grated. Ricotta affumicata is smoked. Ricotta infornata is baked. Each is a further process. When a recipe says ricotta and does not specify, it means the fresh, moist, white cheese. Cottage cheese is not this process. Cottage is a cut curd of milk, kept in lumps. The taste and the grain are different.
Taste
Fresh ricotta tastes sweet, milky, and clean, with a very fine grain. Sheep's-milk ricotta tastes a little richer and more of sheep. Cow's-milk ricotta is milder. It should not taste sour, bitter, or cheesy in an aged way. The texture should be moist. A dry, yellowing tub has lost what the cheese is for.
Salted ricotta tastes salty and denser, a seasoning rather than a cream. If a fresh ricotta tastes like nothing and feels rubbery, it was overworked or overheated in the factory. If it tastes sharply sour, it has begun to ferment. Fresh ricotta does not gain charm from extra days.
In the kitchen
Use fresh ricotta in lasagna, stuffed pasta, and cheesecake, and on toast with olive oil, honey, or a grated hard cheese. Drain it first if a filling must be thick. Let it sit in a sieve. The watery part will leave, and the cheese will taste fuller. Sheep ricotta is the one for a traditional cannoli or cassata when you can find it. Cow ricotta is the usual one for a weeknight baked pasta.
Do not ask fresh ricotta to melt into strings. It softens and can weep. Do not grate it over a salad. That is the job of ricotta salata, or of Parmesan. Cottage cheese can stand in for a baked filling if you accept the lumps. Mascarpone can stand in for a sweet, smooth cream if you accept a much richer, less grainy result. Neither is ricotta.
Around the world
Italy uses ricotta across regions, in savory north-Italian filled pasta and in southern and Sicilian sweets. The United States adopted the cow's-milk tub as an ordinary grocery item, largely through lasagna and cheesecake. Other whey cheeses exist under other names, from Norway to the Mediterranean.
Buy the form the dish needs. Fresh for fillings and for eating with a spoon. Salata for grating over a hot vegetable. A baked or smoked ricotta is a specialty, not the default. When an Italian recipe specifies ricotta di pecora, it wants sheep. A cow's-milk tub will make the dish milder. For many home recipes, that milder tub is exactly what the writer used.