While there exists a large variety of different photosynthetic systems, they all share two common elements - a light gathering antenna and a much smaller reaction center.
Typical antennas consist of numerous light absorbing molecules like chlorophylls.
These pigments are spaced closely enough so that electronic excitation, created on one of them after absorbing light, can be transferred from one pigment to another until it finally reaches the reaction center. The reaction center contains only a few pigments organized in a very special way. When light excitation is trapped on it, it transfers an electron to one side of a cell membrane. The energy of the free electron is used later by the cell to initiate chemical processes. It was the first pigment-protein complex for which the structure was determined with atomic resolution.