ISBN (International Standard Book Number) is an identifier for books, whether of the paper or e-book variety. On occasion it has been applied to other things such as software. It arose as a modification of a 9-digit book code introduced in the 1960s for use in the United Kingdom and Ireland, and began to be used in 1970 (though the U.K. didn't switch from its own earlier system until 1974). It has been standardized as ISO 2108.
The ISBN originally had 10 digits, with the last digit being a checksum to verify the integrity of the codes. Later it was transitioned to a 13 digit code to handle future expansion, though at present the codes in use are convertible between the two systems by adding the prefix digits 978 before the shorter code and changing the checksum to reflect the entire number. Another prefix, 979, has been allocated for future use, and more prefixes may be added in the future.
13-digit ISBNs can be represented directly on EAN-13 bar codes for retail sale. The 978 and 979 prefixes are not in use in retail coding for any non-ISBN items; usually the first digits of an EAN-13 code represent a country, so 978 has been referred to as the "virtual country" "Bookland".
After the prefix, ISBNs have a country code, then the remainder of the code is assigned in blocks to publishers who pay a registration fee and can then assign the individual codes to whatever books they wish. The final digit is a checksum; since the checksum algorithm can produce "10" as a sum, the letter "X" is used in this case, so ISBNs aren't always fully numeric. They are intended to be permanently unique, so they should not be re-assigned to new books even if the original ones are out of print. Publishers are also supposed to assign separate ISBNs for each edition of a book, including separate numbers for different versions such as hardbound, paperback, and e-book.
Serials (such as magazines and journals) use ISSNs to number the entire series. Some of them (such as periodically-issued reference books) also use ISBNs for the individual volumes of the series since they are sold separately as books as well as by subscription, so they have both ISBNs and ISSNs.
Mediawiki, the software used in many wikis including Wikipedia and the File Formats Wiki, has a feature that lets ISBNs be linked automatically to a page that provides various options to find the referenced book in libraries and booksellers.