Actually, the best way to put it would be if the current is too large or too small.
Fuses and circuit breakers are both devices that "break" if the current flowing through them is too large.
A fuse is usually a thin piece of wire that will melt and break the circuit if too large a current flows through it. The thicker the fuse wire, the more the current needed to melt it and the higher the "rating" of the fuse. The function of the fuse is to protect the wiring and the equipment.
A circuit breaker is an electronic circuit that will switch off the current if it is too great. A circuit breaker is designed to operate much more quicker than a fuse and so the damage is less because the current is switched off in a much shorter time.