This is a new one that is by invitation only for now.
http://www.pinterest.comAll it really is is "pinning" pictures of things that you like from all over the web onto "boards" that you create. So, for example you could have a "nursery decor" board and pin pictures of items you like related to nursery decor to it.
The reason it can be great for your Etsy shop is that people can see what you pin if you want them to and if you post pictures from your own shop, the link will be there.
Without trying to get complicated, you bookmark Pinterest and whenever you come across a picture online that you like you can just pin it right from the picture without actually having to save the image or anything. So, the website you got it from is there (and clickable) and you can use tags for searches.
Good way to get people to your shop!
If you'd like an invite let me know your email address and I'll send you one. It's easy to figure out once you're on there!