Curd is a dairy product obtained by curdling (coagulating) milk with rennet or an edible acidic substance such as lemon juice or vinegar and then draining off the liquid portion (called whey). Milk that has been left to sour (raw milk alone or pasteurized milk with added lactic acid bacteria) will also naturally produce curds, and sour milk cheese is produced this way. The increased acidity causes the milk proteins (casein) to tangle into solid masses, or "curds". The rest, which contains only whey proteins, is the whey. In cow's milk, 80% of the proteins are caseins. Curd products vary by region and include cottage cheese, quark (both curdled by bacteria and sometimes also rennet) and paneer (curdled with lemon juice). The word can also refer to a non-dairy substance of similar appearance or consistency, though in these cases a modifier or the word curdled is generally used (e.g. bean curd, lemon curd, or curdled eggs).

In Asia, curd is essentially a vegetarian preparation using yeast to ferment the milk. In the Indian subcontinent, buffalo milk is used for curd due to its higher fat content making a thicker curd. The quality of curd depends on the starter used. The time taken to curdle also varies with the seasons taking less than 6 hours in hot weather and up to 16 hours in cold weather. In the industry, an optimal temperature of 43°C for 4-6 hours is used for preparation. However in India, the word 'curd' is used to mean yoghurt. In India, it is common practice to finish any meal with curd or buttermilk. In Tamil Nadu, South India, it is customary to have few cups of 'curd rice', made by mixing rice and yoghurt. And, in North India, lassi is consumed. It is generally accepted to cool the body in tropical climates, counter the generally spicy cuisine of India and is nutritious.

Cheese curds are popular in some French-speaking regions of Canada such as Quebec and parts of Ontario as well as in the Midwest of the United States. They are freshly made morsels of cheddar cheese before being pressed and aged. In Quebec, they are popularly served with french fries and gravy as poutine. In some parts of the U.S., they are breaded and fried or are eaten straight. Fresh cheese curds squeak against the teeth as they are bitten. There are also many popular varieties besides cheddar, such as white cheeses and flavored cheeses (pepper, garlic, butter, lemon, etc). The cheeses themselves are not flavored but rather lightly coated with a powdered flavor, natural or not, similar to potato chips.