What is the difference between a circuit breaker and a GFCI?

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and interrupt current flow. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city

A residual-current device (RCD), or residual-current circuit breaker (RCCB) or residual twin-direct current couplet (R2D2), is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance may indicate current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions. RCCBs are designed to disconnect quickly enough to prevent injury caused by such shocks. They are not intended to provide protection against overcurrent (overload) or short-circuit conditions

A circuit breaker is a mechanical fuse in essence.
It replaced the old fashioned fuse wire in the distribution board when the live and neural came in to direct contact or the load on the circuit exceeded the current of the fuse (or switch).
A GFCI is a device that sense a leak in the circuit between the lived conductor and the return neutral conductor.
A leak could be for example when the leak passes through a human to earth.

A Circuit breaker can be operated both on Load or off load while a GFCI can only be operated on off load

CB and GFCI both interrupt the circuit current But GFCI is more sensitive and smarter as CB will work only when the current is over it's capacity and it happens practically due to Overload or short circuit.
But GFCI compare the currents to identify any leakage from the circuit and usually used in the places composed to water like swimming pools and bathrooms.

A circuit breaker will operate when "circuit current overload",,,and GFCI operate when difference of current consume by load and current coming from load is more than breaker's setting.
it means if GFCI found any current leaks in circuit it will operate.

Ground Fault Circuit Interrupter Circuit Breaker GFCI CB it is one type of the circuit breakers, and it used usually to power GFCI outlet, and GFCI outlet used near the water source like toilet and kitchen.
The Circuit Breaker will turn the circuit off when there is an overload current.
The GFCI will turn the circuit off when there is an overload current and when there is a current leak.
The GFCI monitors the current going to the Load and compares it to the current coming from the Load.
If there is any difference between the two, the GFCI will turn the circuit off.
The GFCI is very sensitive.
It will trip when there is only a4 to6 milliamp difference between the current going out and the current coming back.
Best regards

CB interrupt the circuit in case of over load or a short circuit from phase or phase or phase to ground , but GFCI sense the current in ground path even a tiny Amps, which usually not being sensed by CB , it keeps your Ground Path Health.

A circuit breaker will operate when "circuit current overload",,,and GFCI operate when difference of current consume by load and current coming from load is more than breaker's setting.
it means if GFCI found any current leaks in circuit it will operate.