In the actual coating process, the coating chamber will inevitably contain micron sized particles, dust and defects and pollution caused in the processing process of the substrate surface. Even after cleaning, they can not be completely eliminated, and may bring new defects and pollution. Impurities, bubbles and micro arc discharge in cathode target also would produce particles. These factors will directly lead to the formation of defects in the film growth. In this paper, the causes and classification of film defects and their effects on different applications are reviewed in detail. According to the formation cause and micro morphology, the defects can be roughly divided into four types:1) flake like structure, 2) pit structure, 3) nodular spherical drop cone structure, 4) pinhole/pore structure. The ratio of different types of defects changes with the environmental factors and their spatial distribution. Defects destroy the integrity of the film, thus significantly affecting the film performance, such as reducing the abrasive resistance of the hard films, and causing the corrosion resistant films lose their protection function as the corrosive medium can reach the surface of the substrate through the defects. For the superconducting films, their residual resistance can be increased greatly when the defect size is much larger than the coherent length of the copper electron pair. Last, the effect of gas pressure, bias voltage, discharge voltage, deposition time, substrate mounting angle, shielding structures in the coating chamber, and others on the defect density are summarized, thus indicating that the defect density can be reduced and the film quality can be improved by optimizing the above parameters.