What is the purpose of a book's ISBN?

The International Standard Book Number (ISBN) is a 13-digit number that serves the purpose of a unique identifier for books and book-like products published internationally. The ISBN is intended for a monographic publication: text that stands on its own as a product, whether printed, audio, or electronic. When participating in the ISBN standard, publishers and self-publishers are required to report all information about titles to which they have assigned ISBNs. For more than thirty years, ISBNs were 10 digits long. On January 1, 2007, the ISBN system switched to a 13-digit format.

Source: American Library Association