Lasers are rated by their gross output power into a class system. In the United States these standards are regulated by OSHA, and in Europe by CE, though the standards are generally the same in specifcation for both bodies.
Class I - Limited to gross output of .5 mW or less and will not emit radiation at known hazard levels. These are the most common type of laser, used widely in CD/DVD pickup assemblies. Class I Lasers are exempt from any excise controls and their manufacture, purchase and distribution is generally unlimited.
Extended expose to Class I laser is generally considered benign, even direct viewing -- in fact, many Class I lasers are so low power that the human eye won't even register them. Still, don't look directly into one, just in case.
Class I.A - A special designation for laser of the same type as Class I, but having a maximum power output of 4 mW. These are commonly seen in Laser Pointers and barcode scanners. Class I.A Laser are commonly marked as Class I, though they will cause corneal damage after 1000 seconds of direct exposure.
Class II - Low-power visible lasers that have gross output above that of Class I/.A levels but at a radiation power still below 1 mW where eye damage will only occur with extended exposure, generally 1/4 to 1/2 seconds. The concept is that the human aversion reaction to bright light (e.g. It hurts so you don't look into the beam) will protect a person from prolonged viewing.
Class IIIA - These will be lasers in the 1-5 mW range (just a little bit more oomph than a class I.A), but that have radiation characteristics that make intrabeam viewing (e.g. Looking into the laser beam) immediately hazardous regardless of the length of time viewed; Immediately hazardous means your eyes will burn out before you can look away.
Class IIIA are further classified as to their danger level: beams that are only dangerous when projected directly in the eyes and beams were the radiance of the dispersion area, where the beam hits something, is also harmful after extended periods. These are given the classifications of Caution and Danger respectively. This means that when working with lasers in the Danger category, which means a beam irradiance of more than 2.5mW cm2, eye protection is mandatory at all times regardless of you likely hood of looking directly into the beam.
Class IIIB - We're getting into the heavy lifters now. These are lasers with outputs of 5 to 500 mW and cause immediate eye damage as well as skin damage with extended (1000-3000 second) exposure. These are one of two kinds of lasers along with Class IV (below) that actually require physical lockouts as they can be used to hurt people.
Class IV - Big daddies. Devices of these types exceed 500mW or 10 Jcm2 and are classified as munitions. These are lasers that are immediately hazardous for any kind of exposure usually due to their UV or raw Lumen output. These are the lasers that are capable of burning things -- even strong things like wood, metal and stone in some special cases.
Ever see Real Genius? The 6 MegaWatt laser they had was waaaayyy over the baseline power of a Class IV and those dinky welders goggles they were wearing wouldn't have been enough. With a device that big, the area of diffusion is dangerous to even have in your line of sight, and the beam itself ionizes any atmosphere it comes in contact with. (But it was a movie, and I liked it, so I'm not going to to hold that against it).
As a public service, even though you may think a laser "isn't that bright", if you look directly into it "you're not to bright either". Looking into any laser is considered dumb and if you do, you deserve everything you get,
The actual emitter material of the laser can be made of a variety of substances and the laser will have differing characteristics depending on the substance used. The most common difference is the length of the wavelength, and wave length is very useful in many fields such as communications where the shorter wavelength means more information can be transmitted in a given impulse. Some laser materials and their associated wavelengths (in µmeters) are:
Argon fluoride (UV) -- .193
Krypton chloride (UV) -- .222
Krypton fluoride (UV) -- .248
Xenon chloride (UV) -- .248
Xenon fluoride (UV) -- .308
Helium cadmium (UV) -- .325
Nitrogen (UV) -- .337
Helium cadmium (purple) -- .441
Krypton (blue) -- .476
Argon (blue) -- .488
Copper vapor (green) -- .510
Argon (green) -- .514
Krypton (green) -- .528
Double-Pumped Nd:YAG(*) -- .532
Helium neon (green) -- .543
Krypton (yellow) -- .568
Copper vapor (yellow) -- .570
Helium neon (yellow)(**) -- .594
Helium neon (orange) -- .610
Gold vapor -- .627
Helium neon (red) -- .633
Krypton (red) -- .647
Rohodamine 6G dye -- .570 to .650 (tunable)
Ruby -- .694
Gallium arsenide (NIR(***)) -- .840
Nd:YAG (NIR) -- 1.064
Helium neon (NIR) -- 1.15
Erbium (NIR) -- 1.504
Helium neon (NIR) -- 3.39
Hydrogen fluoride (NIR) -- 2.70
Carbon dioxide (FIR(****)) -- 9.6
Carbon dioxide (FIR) -- 10.6
* - Thinkgeek sells a green laser pointer based in this system
** - Most common laser for CD and DVD pickup assemblies, though now supplanted buy laser diodes of the same wavelength
*** - Near-Spectrum Infrared, 0.700-1.400 µm
**** - Far-Spectrum Infrared, >1.400 µm
Information culled from OSHA Laser standards manual (section 3, chapter 6), World Book Encyclopaedia and an ex Bell Labs Employee