What is a sheep dog?

Schipperke dog
The sheep dog or shepherd dog is any of several breeds of dogs used to tend sheep. Among the duties of the sheep dog are driving sheep to pasture and guarding them in both the field and fold. The sheep dog is sometimes also employed in tending cattle. The principal varieties of sheep dog are the collie, German shepherd dog, old English sheep dog, schipperke, Belgian sheep dog, and Shetland sheep dog.