Postbiotics, as the metabolic products and cellular components of probiotics, possess the characteristics of being non-living yet retaining biological activity. Postbiotics have unique advantages such as high stability, good security, and a clear target of action. In recent years, they have attracted extensive attention due to their potential roles in immune regulation, anti-inflammation, antioxidation, antibacterial activity, and improving intestinal health. This article systematically reviews the composition of postbiotics and their diversity in fermented foods, with a focus on the impact of different inactivation methods (thermal and non-thermal inactivation) on their biological activities. Many studies have shown that the choice of inactivation method directly affects the immune regulation, anti-inflammatory, and antioxidative functions of postbiotics. Additionally, this review summarizes the application potential of postbiotics in the food industry, the field of medicine and food homology, pet food, and animal breeding, and points out the challenges existing in current research. Future studies need to focus on optimizing inactivation methods to maximize the biological efficacy of postbiotics, thereby promoting the precise application of postbiotics in various fields.