An ISBN is a 10 or 13-digit number that is unique to a particular book. It is used to identify books when they are being bought and sold, and to track them when they are being shipped.
The number can be divided into four parts: the country code, the publisher code, the book code, and the check digit. The country code is used to identify the country in which the book was published. The publisher code is used to identify the publisher. The book code is used to identify the specific book. The check digit is used to verify the accuracy of the other digits in the ISBN.
ISBNs were first introduced in 1970, and they have been required for all books published in the United States since 1974. Other countries have since adopted the use of ISBNs, and they are now used worldwide.