How do you use social bookmarking sites like Digg?

I've had the button left on my websites for quite some time, but I've never really used these sites or been a member. I know you're supposed to submit articles you find interesting, but what kinds of articles do people over there actually like? When should I submit an article, and is being a member on these kinds of sites just about things like agreeing to other articles people have found online and such? I never really just understood this whole thing.