A bolt is a type of fastener, typically made from metal, which is used to join two or more pieces of material together. Bolts are usually tightened using a wrench or spanner. There are many different types of bolts, each with their own specific purpose. For example, there are carriage bolts which have a smooth, rounded head and are used for joining wood; there are also hex bolts which have a hexagonal head and are used for joining metal.