Basically, you collect pictures from the internet onto boards.
It sounded totally pointless to me until I started trying it out. Now I have a board for costume ideas, another for funny pictures I want to show Brian, another for pictures that I might use later, another for dresses I like that I might want to get for a wedding in October, etc. I think it's better than just bookmarking for things that are more visual.
It's by invite only, but let me know if you want an invite.