Ducting is exceptional super-refraction. Super-refraction occurs when the trajectory of a radar beam bends towards the earth's surface more than normal. In other words, the rate the elevation of the radar beam changes with distance away from the radar is less than normal. The radar beam will tend to increase in height above the earth's surface when moving away from the radar site because of the earth's curvature. In a super-refraction situation, the radar beam could be increasing at a lesser rate with height than normal as the beam moves away from the radar site or the beam could even be bending back down and getting closer to the earth's surface in spite of the earth's curvature. It is ducting when the radar beam actually bends closer the earth's surface with distance away from the radar. The bending could be strong enough for the radar beam to bounce off the earth's surface.
Ducting is caused by strong low level inversions (temperature increases with height). Ducting can also occur when a strong cap (EML) of warm and dry air exists in the lower troposphere above very moist air. Ducting causes the radar to be able to sample much further distances than normal. Ducting increases ground clutter also since the radar beam remains closer to the earth's surface for a greater distance and can even bend into the earth's surface. Ducting is more common in the morning hours since this time of the day experiences the strongest low-level inversions (due to cooling of earth's surface through longwave radiation emission) but ducting can also occur anytime a strong cap exists in the lower troposphere.
The advantages of ducting are increased radar range, being able to sample storms further from radar, and being able to sample lower elevations within storms further from radar. The disadvantages of ducting are increased ground clutter and increased anomalous propagation (due to radar beams bouncing energy back from hitting earth's surface or sampling storms beyond the radar's maximum unambiguous range).