What accounts for the different breeds of domesticated dogs?

Variations in breeding over the years. Some dogs were bred to be short and small to hunt ground hogs or badgers, others where bred to do well in the cold and run long distances such as sled dogs. By breding certain dogs with certain characteristics over time you change the make up of the animal. If humans had never interfered with natural canine breeding we would not have nearly as many breeds as we do now. A example is the new "designer" breeds such as Labordoodles or Pickadoodles or what have you.