Covalent organic frameworks (COFs) have emerged as an interesting class of crystalline porous materials with desirable properties (such as highly ordered porosity, structural versatility, high chemical and thermal stabilities, and easy surface modification) and a wide range of potential applications. This critical and important research work aims to provide insights into the design strategies and synthetic methodologies for COFs. Unlike previous reviews on COFs, this research paper also focuses on the application areas and functionalization of COFs, which are important for understanding the physical and chemical properties of COFs required for practical applications. Furthermore, this review highlights the applications of COFs in various fields, including catalysis, photovoltaic devices, sensors, supercapacitors, wastewater treatment, biomedicine, chromatographic and spectroscopic analyses, and gas separation and storage.