The definition of a Category IIIB approach is a precision approach and landing with no decision height or a decision height lower than 50 feet (15 meters) and a runway visual range less than 700 feet (200 meters) but not less than 150 feet (50 meters). About the only approach any hairier than that is the IIIC approach, which is a precision approach and landing with no decision height and no runway visual range limitation. In the video below a Boeing 767 performs a minimums Cat IIIB landing at Milan in Italy.
Local weather at some airports frequently creates Category II and Category III approach conditions. Fog and blowing snow are the most common causes. The Instrument Landing System (ILS) is a ground-based instrument approach system which provides precision guidance to an aircraft approaching and landing on the runway. The ILS consists of the localizer, which indicates landing aircraft heading, the glide scope, which guides altitude, and the airfield lighting system. The pilot controls the aircraft so that the glide slope indicator and localizer needle remain centered on the cockpit display to land the aircraft. The pilot can then visually identify the runway either by pavement or with the assistance of an associated approach lighting system.