here is the official definition of Bookcrossing as per dictionary
bookcrossing
n. the practice of leaving a book in a public place to be picked up and read by others, who then do likewise.
Bookcrossing is a huge international movement which has currently some
644,948 members, we all register our books on the Bookcrossing site, so each book has an individual number and its wonderful journey can be tracked down when the finder leaves a message to say it has been found, what they thought of the book etc.
it has enabled me to read many books that i would have normally not come across, made lots of new friends (locally and internationally), going to conventions etc.