Cereals belong to the Family Gramineae (= Poaceae). They constitute the staple food for a large part of the human population. All cereals have some features in common: the inflorescence is always composed of spikelets (Grains and pseudo-cereals 3) with highly reduced flowers (florets), with 3 or 6 stamens and an ovary with two feathery stigmas. Petals are absent, only one or two small scales, called lemma and palea, are present. There is a characteristic fruit, the caryopsis, with the wall of the seed fused with the ovary wall to form the husk. Tis type of fruit is called grain. These grains consist for the larger part of starch, only the germ and the husks contain proteins.

The most important cereals are:
- wheat, Triticum aestivum, the most important food plant of the world, mainly used for bread, pastas and bakery products;
- rice, Oryza sativa, the staple food in large areas in Asia;
- maize, Zea mays, the larger part is used as cattle fodder;
- barley, Hordeum vulgare, used as ingredient for beer making and as cattle fodder;
- oat, Avena sativa, mainly used as cattle fodder;
- rye, Secale cereale, cultivated in severe climates for bread and cattle fodder;
- sorghum, great millet, Sorghum bicolor, the most important cereal in China, India and Africa;
- millet species, like
- finger millet, Eleusine coracana;
- bulrush millet, pearl millet, spiked millet or cattail millet, Pennisetum typhoides;
- common millet, proso millet, hog millet, broom-corn millet or Russian millet, Panicum miliaceum;
- and the fast growing Japanese millet, Echinochloa frumentacea (see Panicum miliaceum);
all are grown in arid areas in Africa, Asia and South-America.

Some species are only important for local use, like teff, Eragrostis tef, in Ethiopia; and foxtail millet, Setaria italica, in Japan and India.

Local conditions dictate what kind of cereal is cultivated (Grains and pseudo-cereals 2). In the temperate zones wheat is the most important, whereas in more severe climates oat, barley and rye take that place. In humid and warm areas rice is the most important species, followed by maize. Of the area under cultivation in warm and dry areas half is occupied by cereals (Grains and pseudo-cereals 1).

Crops that do not belong to the grass family, but whose fruits and seeds are grinded into meal, are known as pseudo-cereals. Examples are:
- buckwheat, Fagopyrum esculentum;
- amaranths, Amaranthus sp.;
- quinoa, Chenopodium quinoa (see Spinacea oleracea).