A letter of credit is a payment mechanism where a bank guarantees payment to the seller if certain terms and conditions are met. It is issued by the buyer's bank and advises the seller that payment will be made upon presentation of compliant documents. The main parties are the applicant/buyer, beneficiary/seller, issuing bank, advising/confirming bank, and nominated/negotiating bank. The process involves the issuing bank opening the letter of credit at the buyer's request. The advising bank then notifies the seller. If documents are compliant, the nominated bank pays the seller and claims reimbursement from the reimbursing bank.