An International Securities Identification Number (ISIN) uniquely identifies a security. Its structure is defined in ISO 6166. Securities for which ISINs are issued include bonds, notes, certificates of deposit, commercial paper, stocks and warrants. The ISIN code is a 12-character alpha-numerical code that does not contain information characterizing financial instruments but serves for uniform identification of a security at trading and settlement. 

Wikipedia: http://en.wikipedia.org/wiki/International_Securities_Identification_Number