Photosynthesis is a process used by plants, some protists, and some bacteria to make food. It makes glucose (a kind of sugar) and oxygen from carbon dioxide in the air and water. Cells (very small parts that make up a plant or animal) use it to create energy to live.
Photosynthesis starts when light touches a green part of a plant. Each green cell has small parts called chloroplasts inside of them. Single photons of light hit green chemicals in the chloroplast, causing an enzyme to break apart water. Water, when broken, makes oxygen, hydrogen, and energy. The energy is used by a protein that lets light into the cell. The light cannot get out of the cell until another protein lets them, and this protein uses the energy from the escaping light to make ATP. While this is happening, the hydrogen and oxygen join broken up carbon dioxide to form glucose. This glucose is later used in cellular respiration to form more ATP.
Photosynthesis can only occur between 32-95 degrees F (0-50 degrees C), and is most effective at room temperature (72 degrees F, 25 degrees C). Photosynthesis does not work at night or during cloudy days, because not enough light hits the plants.
See also: Photosynthesis Experiments & Background Information
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)