The ISBN (International Standard Book Number) is a unique number assigned to a book title by its publisher for tracking and ordering purposes. An example is:
which is the ISBN for Simon Singh's The Code Book, a book about crypography.
Publishers, retailers, libraries, and readers use the ISBN as a way to specify a particular book without confusion.
The principles and procedures for international standard book numbering are contained in the International Organization for Standardization's Recommendation 2108.