According to application purposes concerning focusability of laser energy, three kinds of lasers, i. e., the annular lasers generated from unstable resonators, Gaussian lasers generated from stable resonators and combined lasers of several Gaussian lasers are taken as analyzing objects. The advantages and drawbacks of beam quality parameters available are discussed, and some key issues on definition of beam quality are calculated and analyzed. The results show that, the parameter of laser brightness or laser energy density on the target is a very important standard for evaluating the performances of laser systems, which is suitable for measuring performance of the same system, as well as different systems objectively.