501(c)3 tax status

Can anyone explain (US) 501©3 tax status to me? I know it is applicable to non-profit organizations, but what are the exact requirements for said status and are there any special circumstances for environmental groups?

I should add… if anyone can explain 501©4 to me too, that would be great…

Ask and ye shall receive…here you go!

Start here:

http://www.irs.gov/charities/article/0,,id=96099,00.html

Whoa, weird. Sorry about the double link.

Lesson of the day for me: hit refresh before posting after sitting on a thread for a while.