The International Bank Account Number (IBAN)   is an internationally agreed means of identifying bank accounts across national borders with a reduced risk of propagating transcription errors.

The IBAN consists of up to 34 alphanumeric characters:

•   first the two-letter ISO 3166-1 alpha-2 country code,
•   then two check digits, and
•   finally a country-specific Basic Bank Account Number (BBAN).

The check digits enable a sanity check of the bank account number to confirm its integrity even before submitting a transaction.

Details of the algorithm can be found on the Wikipedia page.