Passive solar is yet another way to harness the power from the sun. Passive solar uses sunlight for energy.
Generally passive solar gain comes through the windows and is stored in some kind of thermal mass(great masses: stone floor or wall).
Other types of passive solar include an exposed glass water heater or a solar furnace.
The left diagram shows a thermal mass as either the floor coverings, a wall, cabinet or countertop. There are several ways to collect the heat for later distribution (rock is great because it gets hot and then radiates at night). Also shown(not exact) the difference in where the sun is in the winter or summer. In Arizona, the sun is higher in the summer and lower in the winter. For effective solar gain you want to take advantage of this. The design needs to be calculated to be maximized, so its warm in the winter and cool in the summer.