Message-Driven beans are just a new way of describing a JMS message. This has been formalized with the EJB 2.0 specification.
A Message-Driven bean is an enterprise bean that allows J2EE applications to process messages asynchronously.
A Message-Driven bean is an enterprise bean that allows J2EE applications to process messages synchronously.
A Message-Driven bean is used when you need to interact with messaging systems other than JMS.
A Message-Driven bean is like a Session Bean except that if the server is busy it will put the Message-Driven beans' method requests in a queue and execute them one at a time to reduce the load on the Application Server.