The lasers can be classified on the basis of state as explained below:
a) State of laser medium: According to the state of laser medium, we have solid state lasers like ruby laser,gas lasers like He-Ne laser and carbon dioxide laser, semiconductor laser and liquid laser like dye laser.
b) Mechanism of pumping: According to it, we have optical pumping based lasers like ruby laser, electric discharge based lasers like He-Ne laser. Pumping can also be done through chemical reaction.
c) Nature of output: According to nature of output, we have pulsed lasers like ruby laser and continuous wave lasers like He-Ne laser.
d) Spectral region (wavelength) of output: According to it, we have ultra violet, visible and infrared lasers.
Note: Do you know what is pumping or how atoms/electrons are excited?