An outlet that needs to be replaced is usually a result of an impact. Outlets in open, high traffic areas often suffer regular abuse, particularly in public places where sweepers or buffers are being used. Outlets that provide power to high amperage appliances such as microwaves will occasionally need to be replaced as the internal contacts become less reliable due to overheating.
Basic outlets generally have five wires unless the outlet in question happens to be the last one on the circuit. Aside from the green ground, there will be a black and white wire feeding the top or bottom of the outlet and another white and black leaving the outlet providing power to the next outlet in the circuit.