Letter of credit is a form guarantee given to the foriegn supplier.
Usually foriegn suppliers as a condition of supply ask for letter of credit. Once the receipt of suppliers is acknowledged by importer, then exporter, along other documents, presents letter of credit, to his banker. That bank collects the amount from the bank which has issued letter of credit. Importer bank is bound to pay the money wether or not importer has money in his account because it guaranteed payment to exporter by issuing LC.
Generally, LC is given on deposit of margin money and LC charges are collected by bank.