Phase change materials (PCMs) store and release energy in the phase change processes. In recent years, PCMs have gained increasing attention due to their excellent properties such as high latent heat storage capacity, appropriate solid-liquid phase change temperature, thermal reliability, and low cost. Herein, classification, characteristics, and evaluation criteria of organic/composite PCMs are systematically illustrated, and some typical preparation methods are introduced, such as in-situ polymerization, interfacial polymerization, spray drying method, porous materials adsorption method, sol-gel method, melt-impregnated and mixing method, electrospinning method, vacuum infiltration and ultrasonic method are introduced. In addition, this review presents some applications of PCMs in the utilization of solar energy, building materials, air conditioning, industrial waste heat recovery, and military camouflage and stealth. Finally, the development tendency of PCMs is discussed.