Allium species occur in temperate climates of the northern hemisphere, except for a few species occurring in Chile (as Allium juncifolium), Brazil (Allium sellovianum) or tropical Africa (Allium spathaceum). They can vary in height between 5 cm and 150 cm. The flowers form an umbel at the top of a leafless stalk. The bulbs vary in size between species, from very small (around 2–3 mm in diameter) to rather big (8–10 cm). Some species (such as Welsh onion, A. fistulosum) develop thickened leaf-bases rather than forming bulbs as such.
Most bulbous alliums increase by forming little bulbs or "offsets" around the old one, as well as by seed. Several species can form many bulbils (tiny bulbs) in the flowerhead; in the so-called "tree onion" (A. cepa Proliferum Group) the bulbils are few, but large enough to be used for pickling.
Members of the genus include many valued vegetables such as onions, shallots, leeks and herbs such as garlic and chives. A strong "oniony" odor is characteristic of the whole genus, but not all members are equally flavorful.
Some Allium species, including A. cristophii and A. giganteum, are used as border plants for their flowers, and their "architectural" qualities. Several hybrids have been bred, or selected, with rich purple flowers. Allium hollandicum 'Purple Sensation' is one of the most popular and has been given an Award of Garden Merit (H4). By contrast, other species (such as the invasive Allium triquetrum) can become troublesome garden weeds.
Various Allium species are used as food plants by the larvae of some Lepidoptera including Cabbage Moth, Common Swift moth (recorded on garlic), Garden Dart moth, Large Yellow Underwing moth, Nutmeg moth, Setaceous Hebrew Character moth, Turnip Moth and Schinia rosea, a moth which feeds exclusively on Allium sp.