What it is
Pinterest is an image board where you can "pin" images you like, including your own stuff. The idea is to use it as a thought map or online keepsake box for images and videos. There's a facility to post comments, like and follow other people's work, and it posts to Facebook, Twitter, and email. You can also embed a pin on your blog or website if you want to. You can't G+ it yet. They don't like you to use it "purely as a tool for self-promotion," so be sure to like and pin other stuff as well as your own.
How to use it
Visit the website and click the red "request invite" button. Some time later you'll receive an invitation. Go to www.pinterest.com/about/goodies to get the "pin it" button to add to your toolbar on your Firefox or Cometbird browser. When you're browsing the internet and happen to come across a picture you like, click the "pin it" button. Pinterest gives you the option to tweet or share on Facebook when you pin something. It's not yet got a Google + button but I'm sure they're working on that. Create boards to categorize your images. They also take videos.
Using it for marketing
Pin your own stuff, but not to excess. Be sure to repin other people's images and click the like button generously in order to get likes in return. Check their terms and conditions if you want to be more blatant than I am but be careful to respect copyright.
Link to your own blog and social media accounts.
Add pictures that complement yours in order to promote them. E.g. if you sell shoes, you can pin images of places you can wear them.
Invite people to make comments and engage with them as much as possible.
Pinterest won't work for you if you're impatient or don't want to engage with your audience. The idea is marketing via the back door, not an in-yer-face promo blitz.
You want to cross-post to Twitter and Facebook as much as possible to get your audience from there in. The big advantage of Pinterest is not that it's a pin board for showing off your stuff: that's your portfolio's job. It's for sharing visual ideas. If you can leverage the power of visual ideas you'll quickly develop a strong brand. If you try to use it to promote yourself and nothing else, you will fail; that's not what it's for. People don't go there to be bombarded with adverts.
Pinterest has massive SEO benefits, particularly if people are following you and sharing ("repinning") your images ("pins"), so it's a good idea to hop in there every day to pin stuff, repin other people's pins and comment on the ones you like best in the hope of getting some reciprocation.
Happy pinning! See you there.