A solar sail works by harnessing the solar pressure much in the same way a conventional sail harnesses the wind. Any source of radiation also exerts pressure, but in most cases, this pressure is too slight to even be noticed in the presence of more powerful forces such as gravity and air pressure. In space, however, the solar pressure is enough to alter the tails of comets and power spacecraft.
The basic concept behind a solar sail is exactly the same as a traditional wind sail. By deploying a sail with an extremely large surface area, a spacecraft can "catch" the solar wind and use it for propulsion. Since the momentum gained from this technique is extremely small, it requires the spacecraft to be as small and light as possible. The Sunjammer mission was designed as a full-scale test of this technology as a method of propulsion.
The concept of radiation pressure dates back to 1619, when Johannes Kepler noted that the tails of comets always pointed away from the sun, regardless of their actual direction of movement. He postulated that the sun's energy must exert a physical force, albeit a very small one. The concept was proven in the laboratory in 1900 by Russian scientist Pyotr Lebedev.