A ground fault circuit interrupter is a safety device installed in bathrooms, and at exterior electrical outlets.  GFCI's have been required in new construction since the 80's and more recently at the kitchen counter area near sinks.

These outlets provide a high degree of safety at minimal cost.  The units cost under $20 and are relatively easy to install.  GFCI's are designed to trip immediately as opposed to a breaker or fuse which will only trip or blow at their maximum rating. 

GFCI's are designed to protect people while fuses and breakers are designed to protect the house wiring only. 

GFCI's should be tested monthly by depressing the test button to trip the power.  Pressing the reset button will reset the power to the outlet and the circuit.

GFCI breakers  can also be installed at the electrical panel, and are typically denoted by a trip button.