The frog is located on the bottom part of the hoof. It is shaped like a triangle between the hoof wall. The frog is pliable and absorbs the impact protecting the joints and bones of the horses leg while moving and standing.

What I was told by my horse trainer friend and I found it interesting that the frog compresses when the horse puts weight on that hoof. This helps push the blood from the hoof and lower leg back up the circulatory system. This action keeps the blood from pooling in the lower extremity allowing the continuous circulation of fresh blood.

More Info: en.wikipedia.org