The nitrogen-vacancy (NV) color center in diamond is a significant physical system for quantum technologies, including quantum metrology, information processing, and communications, as well as various nanotechnologies such as biological and sub-diffraction limit imaging. Despite extensive research over nearly 50 years, many early assertions about the NV center have been refuted, and several unresolved issues remain. This review aims to provide a comprehensive and self-consistent picture of the current understanding of the NV center by extracting key empirical and ab initio results from the extensive literature. The review covers the identification of the physical structure and charge states of the NV center, its electronic structure, vibronic structure, and optical and spin dynamics. It also discusses ongoing issues and potential avenues for resolution, such as the detailed non-radiative decay pathways, the Jahn-Teller effect, temperature variations, and the factors governing charge state concentrations. The review highlights the importance of the NV center in quantum technologies and the need for a thorough understanding of its properties to enhance its applications.The nitrogen-vacancy (NV) color center in diamond is a significant physical system for quantum technologies, including quantum metrology, information processing, and communications, as well as various nanotechnologies such as biological and sub-diffraction limit imaging. Despite extensive research over nearly 50 years, many early assertions about the NV center have been refuted, and several unresolved issues remain. This review aims to provide a comprehensive and self-consistent picture of the current understanding of the NV center by extracting key empirical and ab initio results from the extensive literature. The review covers the identification of the physical structure and charge states of the NV center, its electronic structure, vibronic structure, and optical and spin dynamics. It also discusses ongoing issues and potential avenues for resolution, such as the detailed non-radiative decay pathways, the Jahn-Teller effect, temperature variations, and the factors governing charge state concentrations. The review highlights the importance of the NV center in quantum technologies and the need for a thorough understanding of its properties to enhance its applications.