When Emerson, a scientist, exposed green plants to differing wavelengths of light, he noticed that at wavelengths of greater than 680nm the efficiency of photosynthesis decresed abruptly despite the fact that this is a region of the spectrum where chlorophyll still absorbs light (chlorophyll is the green pigment in plants - it absorbs mainly the red and blue wavelengths from light, leaving the green light to bounce back and hit our eyes). When the plants were exposed to short-wavelength light, (less than 660nm), the efficiency also decreased.

Emerson then exposed the plants to both short and long wavelengths at the same time, causing the efficiency to increase greatly. He concluded that there must be two different photosystems involved in photosynthesis, one driven by short-wavelenth light and one driven by long-wavelength (PS1 and PS2). They work together to enhance efficiency and convert the light energy to forms that can be absorbed by the plant.

The light excites the chlorophyll molecules at the reaction centre and causes an increase in energy. As the molecule becomes less excited, it's energy is transported through a chain of electron carriers to the next photosystem which does much the same thing and produces energy-carrying organic molecules.

It sounds complicated, and, frankly, it is, but when you see the diagrams it make a lot more sense!