A fuse is a device used to keep electrical circuits from overheating?

A fuse is a device used to keep electrical circuits from overheating is not a true statement. A fuse is used to protect the wire of the circuit from an over current being applied to the circuit. This over current could be caused by a couple of scenariOS. Two non insulated conductors touching one another and creating a short circuit. Another cause could be an non insulated conductor touching adjacent grounded equipment. Whatever the cause the fuse will isolate the fault instantaneously.