They comprise 18 species in six genera.
Males are usually larger than the females.
Penguins congregate to breed in 'rookeries', and often spend many months incubating their eggs while their mates are out at sea feeding. They feed on a mixture of fish, squid, and krill. The wing is long and has no covert or quill feathers, and always remains open. The feathers are tiny, with very broad shafts and but little vane or web. The legs of the birds are placed far back, and in the water the feet are stretched out straight behind and held motionless, the wings working rapidly as if being used in flight.