Ducks represent a number of species in the Anatidae family of birds. They do not represent a single uniform group and there are subfamilies in the Anatidae family. Although geese and swans are also in the Anatidae family, they are not ducks. Other birds that are not in the duck family include coots, loons, grebes, and gallinules. Ducks may be found in both fresh and seawater.
The major sub-familes of ducks are:
Anatinae, the dabbling ducks which feed on the surface of the water rather than diving, and
Aythyinae, the diving ducks
Merginae sea ducks
Oxyurinae Stiff-tailed ducks
For more details see DUCKS in www.ComingsBirds.com/Groups