When the electricity is flowing normally, the fuse permits the power to pass unobstructed across its filament, between circuits. If an overload occurs, the filament melts, stopping the flow of electricity. Fuse is a one time use device and melts when excess current passes through it and protects the other devices in the circuit.
MCB or Miniature Circuit Breaker is either electromagnet or a bi-metal strip. In either case when turned on, the breaker allows electrical current to pass from a bottom to an upper terminal across the solenoid or strip.
When the current reaches unsafe levels, the magnetic force of the solenoid becomes so strong that a metal lever within the switch mechanism is thrown, and the current is broken. Alternately, the metal strip bends, throwing the switch and breaking the connection. To reset the flow of electricity after the problem is resolved, the switch can simply be turned back on, reconnecting the circuit.