Original: The ducks are divided between several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered ducks. Ducks are mostly aquatic birds, mostly smaller than the swans and geese, and may be found in both fresh water and sea water.
Simplified: Ducks are closely related to swans and geese, which are in the same family.