Message Driven Beans are business objects whose execution is triggered by messages instead of by method calls.
The Message Driven Bean is used among others to provide a high level ease-of-use abstraction for the lower level JMS (Java Message Service) specification.
It may subscribe to JMS message queues or message topics, which typically happens via the activationConfig attribute of the @MessageDriven annotation.
They were added in EJB to allow event-driven processing.
Unlike session beans, an MDB does not have a client view (Local/Remote/No-interface), i.e. clients cannot look-up an MDB instance. An MDB just listens for any incoming message on, for example, a JMS queue or topic and processes them automatically. Only JMS support is required by the Java EE spec, but Message Driven Beans can support other messaging protocols. Such protocols may be asynchronous but can also be synchronous. Since session beans can also be synchronous or asynchronous, the prime difference between session- and message driven beans is not the synchronicity, but the difference between (object oriented) method calling and messaging.