This question can be answered on various levels. In terms of organisms, photosynthesis occurs in autotrophes that contain the green pigment chlorophyll. On the multicellular level, photosynthesis occurs in the leaves of plants. As mentioned in the above box, chlorophyll must be present. Sunlight is necessary.
Photosynthesis falls under nutrition, autotrophic nutrition to be exact.
Nutrition has to do with how organisms obtain the materials necessary for energy, growth, etc. We call these necessary materials nutients. Well, it's an important component of the Oxygen-Carbon Dioxide Cycle. Organisms that carry-out photosynthesis are like oxygen factories. At the same time they are removing CO 2 from the atmosphere.