The International Securities Identification Number, or ISIN, is a unique 12 character alpha-numeric international code which identifies a specific security (e.g., stock, bond, warrant).  While the ISIN is intended to identify the security for the purposes of trading, clearance and settlement, it does not designate the particular exchange which a security trades and, in fact, a like security trading on multiple exchanges and denominated in different currencies will have the same ISIN on each. 

The ISIN consists of three parts; a two letter country code, a nine character National Security Identifying Number (NSIN) and a single check digit.  Take for instance Apple, Inc. (trading symbol AAPL) which has the following ISIN: US0378331005.  The first two characters (US) represent the country in which this issuer is domiciled.  The next nine (037833100) represent the NSIN which, as this is a US issuer, is a CUSIP.  The final character (5) is the check digit used for error detection.