Here's is a quick overview of what I found. Basically a GFCI outlet cuts off the electricity much faster than a regular circuit breaker.
Circuit breakers designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected.
GFI, or GFCI – Ground Fault Circuit Interrupter device protects us from receiving electric shocks from faults in the electrical devices we use in our home. It works by comparing the input current on the hot side to the output current on the neutral side. If there’s the slightest difference in current, on the order of a few milliamps, then there is current leaking out somewhere, possibly through somebody’s body. To protect us in this situation, the device very quickly cuts off the power supply to the leaking device, within 20-30 milliseconds, greatly reducing any possible human tissue damage from errant current.