Cocker Spaniels are dogs belonging to two breeds of the spaniel dog type: the American Cocker Spaniel and the English Cocker Spaniel, both of which are commonly called simply Cocker Spaniel in their countries of origin. In the early 20th century, Cocker Spaniels also included small hunting Spaniels.
Cocker Spaniels were originally bred as hunting dogs in the United Kingdom, with the term cocker deriving from their use to hunt the Eurasian Woodcock. When the breed was brought to the United States, it was bred to a different standard, which enabled it to specialize in hunting the American Woodcock. Further physical changes were bred into the cocker in the United States during the early part of the 20th century.
From : http://en.wikipedia.org/wiki/Cocker_Spaniel