Pinterest is a new site where people can share and browse through pictures of cool stuff, whatever that may be. It's supposed to be like a virtual cork board, and you can "pin" photos you find on the internet or ones you take yourself onto various "boards" based on whatever categories you assign them.
It sounds a bit silly and unnecessary, but it's actually pretty cool to scroll through and see some really pretty or funny things people have found. You can also subscribe to your friends' boards so you can see what they're pinning, similar to the idea of the facebook newsfeed.
If you're at all into DIY projects or crafts, you'll find tons of good inspiration on there. All of the pictures link back to the sites they came from, so you can go see how they made the items too.
Last I checked you had to apply for an invitation to the site, but that just means you give them your email address and about two days later they send you an invitation to sign up.
Along the same lines is the site Wanelo, which stands for Want-Need-Love. It's pretty much the same thing, actually, but a bit more focused on products you can actually buy. So you can peruse both if you're feeling extra bored.