Hooks are bent or angled fixings which either hang or retain items. The are many different types of hooks some with specific jobs to do while others are more general purpose. Hooks also come in many different materials, plastic hooks are great for hanging lightweight objects and are often self adhesive which makes them very easy to install at home.
Metal hooks are more heavy duty and are often made from either brass, steel, stainless steel or aluminium. Metal hooks are better for hanging heavier items and as such they need more robust fixings than just a self adhesive strip on the back. Cup hooks for example have a screw thread 'built in' and just directly screw in, robe hooks tend to have a plate with holes in which you then screw in. A cleat hook is used to wrap cord around for use with window blinds or anything where a tensioned cord requires restraining to a wall.