What are the different kinds of enterprise beans ?

Session bean

Stateless session bean

An instance of these non-persistent EJBs provides a service without storing an interaction or conversation state between methods.

Any instance can be used for any client.

Stateful session bean

An instance of these non-persistent EJBs maintains state across methods and transactions.

Each instance is associated with a particular client.

Entity bean

An instance of these persistent EJBs represents an object view of the data, usually rows in a database.

They have a primary key as a unique identifier. Entity bean persistence can be either container-managed or bean-managed.

Message-driven bean

An instance of these EJBs is integrated with the Java Message Service (JMS) to provide the ability for message-driven beans to act as a standard JMS message consumer and perform asynchronous processing between the server and the JMS message producer.