A harpoon is a long spear-like instrument used in fishing, whaling, sealing, and other marine hunting to catch large fish or marine mammals such as whales. It accomplishes this task by impaling the target animal and securing it with barb or toggling claws, allowing the fishermen to use a rope or chain attached to the butt of the projectile to catch the animal. A harpoon can also be used as a weapon. (source)