An International Securities Identification Number (ISIN) is an alphanumeric code that uniquely identifies a security.
An ISIN consists of a two character country code, a nine digit number and one check digit.
Each country where ISIN numbers are used has a single national numbering agency to assign numbers using that country code. This guarantees that the same ISIN cannot be issued to two different securities.
ISINs are not the only codes used to identify securities. They are not even always the most useful. One problem is that nine digit numbers are not very memorable. For this reason, most exchanges have other coding systems that are easier for human beings to use (to call up quotes or enter trades), while the computers that handle clearing and settlement are more likely to use ISIN codes.
Other important codes are TIDM (used for UK listed securities), SEDOL (London Stock Exchange Codes), CUISIP (the US and Canadian equivalent of a SEDOL code), Reuters codes, and Bloomberg codes.
For example Marks and Spencer's ordinary shares have the TIDM MKS, Reuters code MKS.L, Bloomberg code MKS LN, SEDOL 3127489 and ISIN GB0031274896. It also has an ADR with a CUISIP of 570912105.
It can be seen from this example that Reuters and Bloomberg codes are based on the TIDM together with an extension to identify the London market, and that the ISIN for UK companies is based on the SEDOL together with a country code (GB) and a check digit (the 6 at the end in the example above).