International Standard Book Number is a unique identifier for commercial publications.
While it was in sporadic use earlier, it was first standardized in 1970 as a 10-digit number (sometimes X can appear in the last position, which is an algorithmic checksum), it was later expanded to 13-digits in 2007 in order to make it more compatible with common point of sale systems for products other than books (products which use the similar 13-digit EANs).
The ISBN is printed on the copyright page but also generally found in the bar-code label on the rear cover or jacket of a book or jacket flap. Occasionally it might be found on the front cover, especially on older paperbacks.
You can search for books by ISBN on Biblio here. For more information on ISBNs, see this article on our blog.
The illustration below shows a typical location of an ISBN on a book, with the barcode section enlarged for legibility:
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