The idea is to share books and encourage their sharing.
Basically what happens, is you have a book. You take that book, and register it at the BookCrossing web site, acquiring a unique number for that book, called a BCID (BookCrossing ID). You then place a label, with this BCID, on the book. This label generally explains the other concepts behind BookCrossing.
Next, you "release" the book. How do you do that? You give it away. Leave it somewhere. Drop it off. Put it in a restaurant. In a library. At work. At school. The label will explain to people why you did it, and encourage them to read the book, comment about it on the BookCrossing website. . . and re-release the book.
With luck, the book will continue going like this. ;-)
I've already made a trip to used book store looking for some of my favorite stories to share, and I'm also getting ready to release any books that I have duplicates of for some reason or another.
I strongly encourage everyone to join BookCrossing and participate in sharing books. ;-)
(Feel free to list me as a referer, too, if you want. ;-)