International Standard Book Number. A unique numeric commercial book identifier administered by the International Organization for Standardization (ISO) under international standard ISO 2108. Most countries and territories adhere to this international standard. Numbers are assigned by country; the included link is to the organization responsible in the United States.
A nine digit standard was created in 1966. This was expanded to 10 digits in 1967 and became an ISO standard in 1970. Nine digit numbers can be converted to 10 digits by preceding the number with a zero (0). The number was expanded to 13 digits in 2007 (Wikipedia).
Though usually accurate, as with many man-instituted technologies, the system is not perfect; occasionally one will find two [or more] titles with the same ISBN, or other errors in the printing of the number.