ICloud assistance and facts help?

Hi,
I was wondering if someone could explain to me what iCloud is. I know for the most part that it is a place that you store data, but does it take up any space on the computer? Also how do you access a file once it is in the "Cloud", as well as how do you put it in there to begin with? How safe is it? Do I want to put important documents (such as tax info) in it or not?
Thank you!
