As for what i am going to use it for: imagine an immense laser show from a pocket sized device (not including a power supply) as this will not be a display that people or aircraft for that matter will be in a direct line of sight with and it will be pointed to the sky so the diffused lighting effect isnt an issue more to the point it will be used to accentuate a fireworks show so there will be plenty of smoke and fog to show the lights 90% of the lasers should be seen about 150ft in the air as the fireworks will be fused to fire at 250ft.
The next question will probably be "why so powerful you dont need something that powerful" well i have always been one to overdo things and i want a very bright and impressive display.
OK, waitaminnit. You say with one breath that diffused lighting won't be a problem and aircraft won't be affected, yet you plan to aim it into the sky through fog?
I may be jumping to conclusions here (lordy, that's been known to happen) but unless you know for a fact (and have the documentation to prove it) that there are no air lanes anywhere near your planned show site, the aircraft thing could be a problem. And what do you think the fog will do to the beam? It will diffuse it.
As others have already said, polarized glasses will definitely not cut the mustard. You need to get protective glasses specifically designed for the wavelengths you're working at.
I don't think anybody has answered your question about what 'nm' means. It stands for 'nanometers', and gives you the wavelength of the light. The longer the wavelength, the lower the frequency. Blue light has a shorter wavelength than green light, which is shorter than yellow, etc. Red has a longer wavelength, and infrared is longer still (the lowest frequencies you might be able to see are in the near infrared). For reference, reds are in the 700 nm range, blues are in the 400 nm range, and everything else you can see falls somewhere in between. Check Wikipedia and Google for more information.
I don't mean to sound all harsh or "dude that is SO dangerous" but there are plenty of issues to be aware of. Someone posted the link to Sam's Laser FAQ; read it. Three times. You really need to know what you're doing to put on a show with something which has this much potential to blind someone. With a decent laser you can detach someone's retina pretty quickly. Build in lots of failsafes.
Torben