Cyanobacteria, green algae, and higher plants provide the major part of molecular O2 of Earth atmosphere via water oxidation of oxygenic photosynthesis. The water-oxidizing complex is a manganese-calcium oxide-based cluster embedded in Photosystem II that oxidizes water with high turnover frequency. The atomic structure and analysis of the Mn-Ca cluster are important in understanding the mechanism of water oxidation and for the design of efficient artificial water-oxidizing catalysts. With this short review, we aim to introduce the basic features of the biological water oxidation to the new-comers in the field. Taking into account the recent structural studies, including a high-resolution, radiation damage-free structure of the water-oxidizing complex, and structures of intermediate S-states revealed by femtosecond X-ray free electron lasers, we discuss the structure and functions of the biologically active site and its implications for the development of inorganic catalysts for solar fuels production.