I still don't really get Pinterest, but I do love looking at illustrator's sites.
It's a virtual pinboard. So you're surfing the web and you see a picture you like, a craft, an illustration, a dress, a rug, you click on your 'pin it' button (which appears on your web browser once you've signed up to pinterest.com) and then it adds that image to your 'pinboard' along with any description you would like to add. You have different boards for different things - I've got one for a book I'm writing to get me in the right frame of mind/setting, another for products I love, another for beautiful art, another for beautiful outdoor photos and one that's just yellow things! I loooooooove it.
Ooh and I forgot to add that you can follow other people's pins, if you like (not people you know necessarily, just random people out there). I follow this one guy's boards who somehow finds the most beautiful photographs from around the world, and then I repin his pins to my boards. Now I'm making it sound complicated... it really isn't!