Both are modes in which consumers can receive messages from Service Bus.
Receive-and-delete process is often called at-most once processing(each message is processed at most once - chances of message loss in case of any failure ). Deletes the messages as soon as it is picked by consumer for processing.
Peek-lock process is often called at-least once processing(each message is processed at least once - less chances of message loss in case of any failure). When the message is picked, a lock is placed on it making it unavailable to other competing consumers and once processing is completed it will invoke Complete() function and message will be removed from queue. If consumer fails to process or does not respond within specified time interval(default 5 mins), Abandon() function will be invoked and message will be available for other consumers.
