A letter of credit is an obligation taken on by a bank to make a payment once certain criteria are met. Once these terms are completed and confirmed, the bank will transfer the funds. This ensures the payment will be made as long as the services are performed.
A bank guarantee, like a line of credit, guarantees a sum of money to a beneficiary. Unlike a line of credit, the sum is only paid if the opposing party does not fulfill the stipulated obligations under the contract. This can be used to essentially insure a buyer or seller from loss or damage due to nonperformance by the other party in a contract.