Here is my basic understanding, along with "what not to do" which I found out on my own.
1. You can download a little button that sits up on your bookmark or menu bar, and when you're browsing around the internet and see something, anything you like, you "pin" it (by clicking the Pin It button). It could be a recipe, a cool lamp, a book, a quote .... and then you can sort them onto Boards. After a while, you can pull up a Board and get a general idea of say, your style. Or things you want to read. Or in my case, buy.
2. You can also go to the Pinterest site and see other people's pins and re-pin them. It's easy to get lost looking for a great knitting pattern or an idea for a backsplash.
3. Your friends can "follow" your pins (and there is a lesson here) by being notified when you pin something. And you can follow them, or anyone else on Pinterest.
So I was reading The Bloggess, who has a pretty snarky, hilarious, and possibly inappropriate sense of humor, and I clicked on a link of things she pinned. Of course I was laughing at those as well, pinned one that had a bad word every other word, not realizing all my friends and followers were notified that I repinned it! Kinda funny, kinda like way back when I joined Facebook and sent an invitation to EVERYONE in my address book. Including my CPA .... school superintendent ..... etc.