Active solar systems use any of a variety of “active” methods to maximize the energy they gain from the sun. These range from simple pumps or fans to motorized retractable awnings, to computerized sun-tracking solar panels or even rotating the whole house to follow the sun. These active systems typically cost more and are prone to break downs.
Passive solar systems are typically less complex and this has made them more popular. In many cases, designing a moderately passive solar home requires no additional cost at all, it is more about putting the normal components in the optimal place and then using the home as the collector. Using natural convection to distribute the warm air is standard but can be improved with minor (or major) adjustments to the geometry of the home. Summer cooling can be enhanced with windows (cross breeze) or by placing openings at different heights (Bernoulli principle) or with a solar chimney. A solar chimney work by solar heating air in the chimney so that it rises faster (boyancy) and increases the draw from the home. Cooler air is drawn in from the perimeter or thru earth tubes to replace the air being sucked out the solar chimney.
In many cases, a system that was designed as a passive system, may work a little better with some help to move the heat around. For instance, a passive home owner may find that his convective loop isn’t moving the air quite as quickly as he would like, and so a ceiling fan or other ducted fan is added to the system. I intend to use my radiant floor heating system to also distribute solar energy from the direct gain portions of my home to the darker corners. If the power goes out, the passive portion of the system still works, the augmented system just works better. In many cases, the small and robust fans used to augment the system are solar powered.
Casually, we might still call that “passive solar”, but I guess we could get picky and call it “augmented passive solar” to differentiate it from the other two categories.