Photosynthesis is the process by which plants and a few types of single-celled organisms use energy from the sun to transform carbon dioxide and water into a storable form of energy: glucose (a type of sugar). Photosynthesis also gives off oxygen as a byproduct. Here is the equation for photosynthesis: 6CO₂ + 6H₂O —› C₆H₁₂O₆ + 6O₂.

One of the most important and impressive aspects of this relatively simple process is that it is responsible for feeding -- either directly or indirectly -- nearly all life forms on Earth. Plants, of course, use some of their own stored energy to support their life processes. First and foremost, the energy from photosynthesis is used by plants for growth and reproduction. Animals, however, evolved long ago to take advantage of the energy that plants store in their tissues. The majority of animals, including humans, rely on the carbohydrates found in plant roots, stems, leaves, and fruits as their primary source of energy. Even animals that eat only meat benefit from plants indirectly. Animals also store energy in their tissues. Thus, the energy a plant-eater acquires when it consumes a plant can be transferred to an animal when it consumes the plant-eater.

Source
http://www.teachersdomain.org/resource/tdc02.sci.life.stru.methusweb/