What Does English "Cocker Spaniel" Mean?

These dogs are called "cockers" because they are used to flush birds from their hiding places, like Woodcock.  Mustaine is a little off in claiming that the American Cocker spaniel is known as the "Cocker spaniel" outside the states.  It is not.  
The English breed is the original.  When specimens were imported to the States, Americans bred it to a different "blueprint" than the UK.  Eventually the two types were so different they were split into two breeds, the English and the American.  These two breeds are recognised in their own right worldwide.

The English Cocker Spaniel is a breed of a gun dog. The English spaniel cocker resembles its American cousin, the American Cocker Spaniel. Outside the Untied States of America the American Cocker Spaniel is simply known as the Cocker Spaniel.

The English Cocker Spaniel has been given the name of a 'Merry Cocker" due to its happy disposition and its continuous tail wagging habit. The cocker is a very well balanced, compact sturdy dog and it has a typical characteristic expression which shows its intelligence and alertness. It has lobular ears and dark eyes. The ears are so long that if they are pulled, should easily reach the tip of its nose.