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
, the diving ducks
Merginae sea ducks
Oxyurinae Stiff-tailed ducks
Tadorinae shelducks

Anatinae: the major genus is
American Black Duck Anas rubripes
Pacific Black Duck or Grey Duck Anas superciliosa
Garganey Anas querquedula
Red Shoveler, Anas platalea
Cape Shoveler, Anas smithii
Australasian Shoveler, Anas rhynchotis
Northern Shoveler, Anas clypeata
American Wigeon Anas Americana
Chilöe Wigeon Anas sibilatrix
Eurasian Wigeon Anas penelope
Gadwall Anas strepera
Falcated Duck
Northern Pintail Anas acuta
White-cheeked Pintail Anas bahamensis
Yellow-billed Pintail Anas georgica
South Georgia Pintail Anas georgica georgica
Eaton's Pintail Anas eatoni
Galapagos Pintail Anas bahamensis
Mallard Anas platyrhynchos
Baikal Teal
Cinnamon Teal Anas cyanoptera
Silver Teal group
Blue-winged teals Anas discors
Green-winged Teal Anas crecca

Other genera are:
Crested Duck Lophonetta specularioides
Muscovy Duck Cairina moschata
White-winged Wood Duck, Cairina scutulata
Mandarin Duck Aix galericulata
Wood Duck Aix sponsa
Green Pygmy Goose Nettapus pulchellus
Cotton Pygmy Goose Nettapus coromandelianus.
African Pygmy Goose Nettapus auritus of sub-Saharan Africa.

Aythyinae: the major genus is:
Canvasback Aythya valisineria
Common Pochard Aythya ferina
Redhead Aythya americana
Ring-necked Duck Aythya collaris
Hardhead Aythya australis
Baer's Pochard Aythya baeri
Ferruginous Duck Aythya nyroca
Madagascar Pochard Aythya innotata
New Zealand Scaup Aythya novaeseelandiae
Tufted Duck Aythya fuligula
Greater Scaup Aythya marila
Lesser Scaup Aythya affinis

Other genera are:
Black-billed Whistling Duck, Dendrocygna arborea
Rosy-billed Pochard Netta peposaca

Merginae Some of the genera are:
Common Goldeneye Bucephala clangula
Barrow's Goldeneye Bucephala islandica
Bufflehead Bucephala albeola
Long-tailed Duck Clangula hyemalis
Harlequin Duck Histrionicus histrionicus
Hooded Merganser Lophodytes cucullatus
Common Merganser Mergus merganser
Red-breasted Merganser  Mergus serrator
Surf Scoter Melanitta perspicillata
White-winged Scoter Melanitta fusca
Black Scoter Melanitta americana
King Eider  Somateria spectabilis
Common Eider Somateria mollissima

Subfamily Oxyrinae some of the genera are:
Masked Duck    Nomonyx dominicus
Ruddy Duck   Oxyura jamaicensis

Subfamily Tadorinae some of the genera are:
Tadorna: shelducks (Europe, Africa, Australasia; 7 species)
Paradise Shelduck Tadorna variegata
Salvadorina: Salvadori's Teal (New Guinea)
Neochen: Orinoco Goose (South America)
Chloephaga: sheldgeese (South America; 5 species)
Hymenolaimus: Blue Duck (New Zealand) Hymenolaimus malacorhynchos
Merganetta: Torrent Duck (Andes mountains, South America) - formerly in "perching ducks"
Tachyeres Steamer Ducks
Flightless Steamer Duck Tachyeres pteneres