According to Reference.com, there is no difference between a pollywog and a tadpole. Both words describe the larval stage of the toad or frog. These amphibians for the most part live in the water during this stage of their lives.
Frogs and toads begin their lives as eggs laid in the water. The eggs hatch into tadpoles. As the tadpole matures, it goes through a metamorphosis by gradually growing legs followed by the arms. The tail of the amphibian is then absorbed. About the same time the animal develops legs, it is also developing lungs, allowing it to live on the land. Late during the tadpole stage, the animal lives near the water's surface and is breathing air. During the final part of the amphibian's life as a tadpole, the small enclosed mouth changes to a mouth as wide as its head.
As tadpoles become frogs or toads, their diet also changes. During the tadpole stage, the amphibian is an herbivore. Algae and plants in the water meet its nutritional needs. However, in the adult stage, it becomes an omnivore, with a diet consisting primarily of insects.
The etymology of both tadpole and pollywog depend on the Middle English word "pol," meaning head. Tadpole uses another Middle English word, "tadde" from which the word toad derives, while pollywog uses "wiglen" meaning to wiggle.