Direct solar is a term used to describe the capture of the sun’s energy to heat the space of a building, its hot water system, or both. It’s used to distinguish these systems from photovoltaic (PV) solar energy systems, which capture the sun’s energy to generate electricity.
Active direct solar systems use a medium (typically water) which is heated in a collector, often on a rooftop, and then circulated to heat the building in either a hot water or forced hot air system. With a passive direct solar system, the sun heats the building directly; this is accomplished with building design features such as south-facing windows, overhangs to shield from the summer sun, and materials with high thermal mass, such as rock.
The U.S. Department of Energy’s EERE website (Energy Efficiency and Renewable Energy) offers a useful introduction to the principles of solar energy. The website of BuildItSolar has many specific tips and suggestions for incorporating solar energy. Energy Planet has hundreds of links to solar energy sites. And the National Renewable Energy Laboratory (NREL) has a very clear explanation of the basic principles of incorporating passive solar energy principles into a building’s design.