Solid-state lasers have lasing (like a ruby) material distributed in a solid matrix. When excited electrically they emit a photons.

Gas lasers (helium and helium-neon, HeNe, are the most common gas lasers) have a primary output of visible red light. CO2 lasers emit energy in the far-infrared, and are used for cutting hard materials.

Excimer lasers use reactive gases, such as chlorine and fluorine, mixed with noble gas(es) such as argon, krypton or xenon. When electrically stimulated, a dimer is produced. When excited, the dimer will produce a laser

Dye lasers use organic dyes in solution or suspension. They can be tuned to emit over a broad range of wavelengths.

Semiconductor lasers, sometimes called diode lasers, are not solid-state lasers. These electronic devices are generally very small and use low power. (Howstuffworks)

Lasers are Classified in to area's depending on how much damage they can do a person. Every laser is to be labeled in the United States. Here are the classifications:

Class I - These lasers cannot emit laser radiation at known hazard levels.

Class I.A. - This is a special designation that applies only to lasers that are "not intended for viewing," such as a supermarket laser scanner. The upper power limit of Class I.A. is 4.0 mW.

Class II - These are low-power visible lasers that emit above Class I levels but at a radiant power not above 1 mW. The concept is that the human aversion reaction to bright light will protect a person.

Class IIIA - These are intermediate-power lasers (cw: 1-5 mW), which are hazardous only for intrabeam viewing. Most pen-like pointing lasers are in this class.

Class IIIB - These are moderate-power lasers.

Class IV - These are high-power lasers (cw: 500 mW, pulsed: 10 J/cm² or the diffuse reflection limit), which are hazardous to view under any condition (directly or diffusely scattered), and are a potential fire hazard and a skin hazard. Significant controls are required of Class IV laser facilities.
(Howstuffworks)