A session bean represents a single client inside the Application Server. To access an application that is deployed on the server, the client invokes the session bean’s methods. The session bean performs work for its client, shielding the client from complexity by executing business tasks inside the server.

• A stateless session bean does not maintain a conversational state for the client. When a client invokes the method of a stateless bean, the bean’s instance variables may contain a state, but only for the duration of the invocation. When the method is finished, the state is no longer retained. Except during method invocation, all instances of a stateless bean are equivalent, allowing the EJB container to assign an instance to any client.
• In a stateful session bean, the instance variables represent the state of a unique client-bean session. Because the client interacts (“talks”) with its bean, this state is often called the conversational state.The state is retained for the duration of the client-bean session. If the client removes the bean or terminates, the session ends and the state disappears.

An entity bean represents a business object in a persistent storage mechanism. Some examples of business objects are customers, orders, and products. In the Application Server, the persistent storage mechanism is a relational database. Typically, each entity bean has an underlying table in a relational database, and each instance of the bean corresponds to a row in that table.A message-driven bean is an enterprise bean that allows J2EE applications to process messages asynchronously. It normally acts as a JMS message listener, which is similar to an event listener except that it receives JMS messages instead of events. The messages can be sent by any J2EE component — an application client, another enterprise bean, or a Web component — or by a JMS application or system that does not use J2EE technology. Message-driven beans can process either JMS messages or other kinds of messages.