Here's how it works: you get an invitation from someone to join or you request an invite. You set up whatever kind of boards you like, the sky's the limit. You pin items to your boards from anywhere on the web. If you see something that someone else has pinned and you like it, you repin it to one of your boards. You can follow the boards of others and vise versa. Check it out here.
This is something that I posted to my 'Quotes' board. It says exactly how I feel.
I repinned this from here.
If you happen to make your way over there, have fun! Check out my boards and if you like them, follow one or all of them. You might even discover a new addiction! Have you discovered StumbleUpon........