You use them to post sites you like so others can find them.
That is for Delicious
That is for digg. I actually read this site.
That is for furl.
That is for facebook. I have a profile there.
That is for Stumble Upon. I use this. They wa they are set up makes it fun because it can be random and highly addictive.
That is for reddit. I read reddit. It is like the everymans digg.
That is for slashdot. News for nerds. You bet I read /.
The idea is that interesting stories, pictures, and blogs get shared with other people.