What does Photovoltaic mean and why is it useful

Photovoltaics

Photovoltaics is the direct conversion of light into electricity at the atomic level. Some materials  
exhibit a property known as the photoelectric effect that causes them to absorb photons of light and release electrons. When these free electrons are captured, an electric current results that can be used as electricity.

The photoelectric effect was first noted by a French physicist, Edmund Bequerel, in 1839, who found that certain materials would produce small amounts of electric current when exposed to light. In 1905, Albert Einstein described the nature of light and the photoelectric effect on which photovoltaic technology is based, for which he later won a Nobel prize in physics. The first photovoltaic module was built by Bell Laboratories in 1954. It was billed as a solar battery and was mostly just a curiosity as it was too expensive to gain widespread use. In the 1960s, the space industry began to make the first serious use of the technology to provide power aboard spacecraft. Through the space programs, the technology advanced, its reliability was established, and the cost began to decline. During the energy crisis in the 1970s, photovoltaic technology gained recognition as a source of power for non-space applications.

Thanks to NASA for putting it so well...you can find the rest if you would like to follow the link.

http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/
http://www.alternative-energy-news.info/technology/solar-power/photo-voltaics/