A grappling hook has three or four sharp, metal hooks curving outwards from a central rod. An eye at the end of the rod allows you to attach a length of rope. A fisherman, ninja, or climber throws a grappling hook to a high cable, tree, or wall, where it's bound to bite into wood or catch on an uneven surface. Then they scale the obstacle by pulling themselves up the rope or swinging from one ledge to another.
Historically, warriors and ninjas used a grappling hook to climb over walls and into trees. Japanese warriors used one type, called a kaginawa, as a climbing aid as well as a weapon. They swung the kaginawa in small circles by holding the rope close to the hook. Working up momentum, they expertly launched it at an enemy to trip them in a tangling rope or impale them with pointed hooks. In China, kung-fu practitioners used the fei zhua, a "flying claw" to reach higher ground on cliffs and buildings. It doubled as a heavy, barbed weapon.
Centuries ago, fisherman on large trawlers would use a grappling hook to swing between ships. If one ship was heading into bay, fisherman chose to 'catch a ride' to land when they were done trawling. Rather than boating or swimming over, the men were safer tying themselves to ropes and hooking onto cables of the ship that held the sails. Then they swung from the sails of one ship to land on the next one. Since grappling hooks resemble several large fishhooks tied together, their origin may lie with the ocean.
Climbers need a modern grappling hook, made with one flat, wide hook, to access mountains and caverns. These are often used in the controlled environment of an interior climbing wall, with ledges, cliffs, and outcroppings called flakes. Their edges are designed to hold a device shaped like the chunky grappling hook. Climbers practice for outdoor mountaineering or enjoy such sport as a hobby.
Today, we are more likely to encounter a grappling hook in a role-playing game than any real life situation. It lends itself to adventures where a character must scale a castle wall, climb a rocky mountain, or move between levels of a building with flair and ease. Knights, gnomes, and superheroes store the self-propelling and self-winding grappling hook in their belt along with other mythical tools.