Basically what you do is sign up for a free account and then post 10 books you have that you don't really read anymore. Posting is very simple, you just have to punch in the ISBN number. Once you've posted 10 books you get 2 free credits, each good for a book. Then you can search through the catalog of over 3,000,000 books and order them. When you order, the person on the other end mails you the book free of charge and it is yours to keep. The trade off is that when someone requests one of your books you do the same, paying for the postage it costs to send. When a person requests one of your books, you get a credit to request another book. It's as easy as that!
I read a LOT and am often disappointed when the library doesn't have what I'm looking for. This is a great source to find what I'm looking for, and best of all, I can choose to keep the book if I want.