Swans are birds of the familyAnatidae, which also includes geese and ducks. Swans are grouped with the closely related geese in the subfamilyAnserinae where they form the tribe Cygnini. Swans usually mate for life, though 'divorce' does sometimes occur, particularly following nesting failure. The number of eggs in each clutch ranges from three to eight.

Swans are the largest members of the duck family Anatidae, and are amongst the largest flying birds. The largest species, can reach length of over 1.5 m (60 inches) and weigh over 15 kg (33 pounds). Their wingspans can be almost 3 m (10 ft). Compared to the closely related geese they are much larger in size and have proportionally larger feet and necks. They also have a patch of unfeathered skin between the eyes and bill in adults. The sexes are alike in plumage, but males are generally bigger and heavier than females.

The legs of swans are normally a dark blackish grey colour. Bill colour varies: the four subarctic species have black bills with varying amounts of yellow, and all the others are patterned red and black.

The swans are generally found in temperate environments, rarely occurring in the tropics.

Swans feed in the water and on land. They are almost entirely herbivorous, although small numbers of aquatic animals may be eaten. In the water food is obtained by up-ending or dabbling, and their diet is composed of the roots, tubers, stems and leaves of aquatic and submerged plants.

Swans form monogamous pair bonds that last for many years, and in some cases these can last for life. The nest is on the ground near water and about a metre across. Unlike many other ducks and geese the male helps with the nest construction. Average egg size (for the mute swan) is 113 x 74 mm, weighing 340 g, in a clutch size of 4 to 7, and an incubation period of 34–45 days. They are the only anatids where the males aid in incubating the eggs.