Harnessing the sun's energy has been a concept that has been around for as long as we have, and even before. After all, every plant and animal basking in the sun is using solar energy in its direct form.
There are many different ways to use the sun's energy. In today's terms, we break it down into two main categories:
Passive Solar Energy is the use of the sun's energy by way of designing to maximize the use of solar radiance. This includes orienting buildings in specific ways, using materials of certain properties, or installing improvements that use the direct heat of the sun. Some examples include, sun tunnels in roofs, solar water heaters for pools, stepping out into the sun from the shade, etc. Passive solar energy is typically thought of in modern terms as using the sun to reduce the need for other energy sources.
Active Solar Energy is where the sun's energy is captured and turned into electrical energy. This is in the form of photovaltaic cells, and even thermal collectors. Active solary energy is what most people typically think of, when they think of solar energy.