Originally, the term "Decoy" referred to a small pond with a long, cone-shaped wicker tunnel.  The dutch called these setups eende kooi - or "duck cages."  

How did it work?  

Well, the duck hunter would wait in ambush for ducks to settle down on his pond.  Once the ducks were sitting there, relaxing and doing whatever it is ducks do, the hunter would set his dog on them, chasing the ducks into the long tunnel to be captured.

While this technique was originally used by the Dutch to catch ducks to take to the market for food, now it is used by ornithologists to catch ducks for ringing.

Decoys have obviously come a long way since those early duck hunting days, but the goal of our modern decoys hasn't changed - to catch a lot of ducks!