As I understand it, you register on the site. You put the info in the inside cover of the book that you like and want to pass on. You leave the book in a public place or wherever you want. Someone finds it. They look in the inside cover and discover the groovy site Bookcrossing. Then they can read the book, comment on the site...and you can even "go hunting!" You click on you country, state and, if you're lucky, town to see where someone has "released a book into the wild" and go hunting!
I love it.