Fuses and circuit breakers are both devices that 'break' if the current through them is too
large.
In its simplest form a fuse is just a piece of wire that heats up as the current through
it increases. Eventually the wire reaches such a high temperature that its melts. Thicker pieces of wire have a lower resistance than thinner ones of the same length and so the heating effect in the thicker wire is less than a thinner one (for the same current) and so thicker wires can stand a bigger current before they break.
The disadvantage a fuse wire is that it takes a finite length of time for
the wire to heat up and break, and in that time damage may be done to the equipment.
Essentially the circuit breaker compares the current in the live and neutral wires. The current in each of the two wires should normally be the same but if the difference between them is greater than a pre-set value (usually about 20 mA) the circuit breaker cuts off the power.
A
circuit breaker is an electronic circuit that will switch off the current if it is too great. Such a circuit is
designed to operate much more quickly than a fuse and so the damage is less because the current
is switched off in a much shorter time.
Circuit breakers are often used to protect the user if
the casing of the equipment becomes live. They break the circuit before the person using the
equipment gets a near fatal shock.