A fuse is an electrical component which is placed in an electrical circuit for protection against electrical faults such as a current surge.
A fuse is an electrical wire which is thick enough to allow the required current for the device to pass through it, but thin enough to melt and break the circuit if too much current flows.
The Earth Wire
Electrical appliances with metal cases usually contain an earth wire. If there is a fault in the appliance and the live wire makes contact with the metal casing, a large current flows from the live wire to the earth wire. This larger current melts the fuse in the live wire breaking the circuit and isolating the appliance from the live wire.
The earth wire and the fuse together protect the user and the appliance.
Some appliances are double-insulated this means they have a plastic insulating case and therefore do not require an earth wire connect to them. Double-insulated appliance such as hair dryers are marked with the symbol: