Auto Defrost refrigerators do not require much intervention to prevent your unit from frosting. An electronic timer controls a mechanism that periodically warms the evaporator, melting the frost. The water is then collected by a pipe and harmlessly drained away out the back of the unit.
Manual Defrost refrigerators require the owner to remove the frost. These units are usually less expensive than auto defrost refrigerators but require more maintenance.
Frost Free refrigerators do not require any automatic or manual intervention. They simply are designed to not allow the buildup of frost in the first place.