IBAN is an abbreviation from International Bank Account Number.
IBAN was created by European Bank's Standards Committee to support payment's maintenance in European Union area.
Polish IBAN consists of 28 chars and digits - it is compilation of full account number (in Polish NRB) and country code (for Poland it is PL).
Example of Polish IBAN according to standards:
PL 12 1234 5678 1234 5678 9012 3456
Full Polish IBAN consist in fact of 4 parts: country code(2 chars), control digits (2 digits), bank's account number(8 digits), customer's account specific number (16 digits).
Providing proper IBAN number and BIC code is necessary if you would like to send foreign transfer as SEPA transfer.