Lasers can be divided into groups according to diffirent criteria

the state of matter of the active medium : solid, liquid, gas, or plasma

the spectral range of the laser wavelength : visible spectrum , Infra-Red (IR) spectrum , etc

the excitation (pumping) method of the active medium : optical pumping , electric pumping , etc

the characteristics of the radiation emitted from the laser

the number of energy levels which participate in the lasing process