This review highlights the synthesis and applications of the carbon nitride (CN_x) family with not only the different carbon/nitrogen ratios but also different basic units and linkages (triazine, triazole, and azo-linkage). Carbon nitride is a polymer semiconductor composed of carbon and nitrogen, with a basic sp²/sp³ unit of aromatic C–N rings. The most commonly synthesized form of graphitic carbon nitride (g-C₃N₄) has a bandgap energy of 2.6–2.7 eV, making it suitable as a visible light-driven photocatalyst. Further, nitrogen atoms at the edges of 2D g-C₃N₄ nanosheets have lone pair electrons, which can act as Lewis base sites in catalytic conversion reactions. The carbon/nitrogen ratio in carbon nitride and its structural features are affected by its precursors and the synthetic process. When the nitrogen content or carbon vacancy is increased in g-C₃N₄, the defects or unbonded nitrogen may act as recombination centers or introduce states within the forbidden gap, which affect the photocatalytic reaction. Contrastingly, the additional lone pair electrons provided by the unbonded nitrogen are advantageous to the catalytic reaction. However, excess nitrogen may result in the formation of unstable C–N–N bonds that weaken its structural stability. The correlation between different units, including triazine, triazole, tri-heptazine, and azo-linkage, in C₃N_x (x = 3–7) was studied to understand its structural features and their activity in photocatalytic and catalytic reactions. In this review, we summarize the latest synthetic approaches of CN_x and discuss the outlooks of these materials for energy and environmental applications.