Ducks are part of the Anatidae family, together with geese and swans. They live in various freshwater and marine habitats across the world, except Antarctica. Freshwater duck species are abundant, but some are threatened by extinction. Species of freshwater ducks include members of the genera Anas, Aythya, Netta, Sarkidiornis, Lophonetta and Oxyura.

Anas is a numerous genus of ducks, including the squawk duck (Anas formosa), the American widgeon (Anas Americana) and the mallard or wild duck (Anas platyrhynchos), which is the ancestor of all types of domesticated ducks. Anas ducks show strong sexual dimorphism (difference in form of male and females of same species), with males exhibiting bright and colorful feathers while females are smaller and often tan, brown or black.

The genus Aythya contains 12 species of ducks, most of them migratory. The lesser scaup (A. affinis) prefers freshwater environments, but is also found in brackish water near estuaries. Other species include the redhead duck (A. Americana), which breeds in western areas of North America; the hard-head duck (A.australis), which is native to Australia; the ring-necked duck (A. collaris); the endangered Baer's pochard (A. baeri), from Asia; and the golden-eyed tufted duck (A. fuligula).

The genus Netta includes the freshwater South-American species southern pochard (Netta erythrophthalma), the red-crested pochard (Netta rufina), the rosybill pochard (Netta peposaca), with males showing a bright red bill, and the critically endangered pink-headed duck (Netta caryophyllacea), native of India. The crested duck (L. specularioides), the only member of the genus Lophonetta, is found in Argentina and Chile.

The knob-billed duck (Sarkidiornis melanotos) is a freshwater species found in Africa, South America and Asia. Male adults have a characteristic bill with a black knob, as well as a white head with black spots, bright iridescent blue to green feathers and a white body. The genus Oxyura includes stiff-tailed ducks, which feature long tails and a large bill in males.