Letters of credit have a long history dating back to ancient instruments used in South Asia called Hundis. They evolved to become the primary mechanism facilitating international trade by substituting the credit of banks for buyers and sellers. There are two main types - commercial letters of credit used for specific transactions, and standby letters covering secondary payment obligations. Letters of credit guarantee payment by a bank to a third party if certain conditions are met, allowing international trade to occur over long distances before goods are received. They have played a crucial role in the development of global commerce and corporations.