Why we need GFCI (Ground Fault Circuit Interrupter) outlets in our kitchen and bathrooms is relatively simple: We need them to keep us from getting shocked.
A normal panel circuit breaker is to protect you from a fire. When a positive and a neutral wire touch each other it creates a short circuit that a large unregulated amount of electricity can go though, which will quickly overload the capability of the wire, making it get hot like a stove element and possibly setting fire to your home. A panel breaker will trip when the voltage going though the wire exceeds the specification of the circuit. This is most often 15 amps.
A GFCI monitors the amperage in both the positive and neutral wire and trips when there is a difference between the two. It does this because when there is a difference in voltage the unaccounted voltage could be coming from an appliance like a hair dryer that has gotten wet and is feeding voltage into the water. You may in turn touch the water and get shocked.
How all those different technologies work internally, I do not know. Of course a fuse works by running the circuit though a filament that will burn up if too much voltage passes though it, but panel breakers and GFCIs are a mystery to me!