Head First EJB

Beans come in three flavors

Entity

Use an entity bean to represent a thing in a persistent store. That almost always means something in a database, where an instance of an entity bean represents a row in a table (although if the database is normalized, the bean might be drawing from rows in multiple tables). A typical entity example is Customer, where one entity might represent Bo Olean (ID# 343) and another entity might represent Trixia Lin (ID# 870).

Message-driven

Use a message-driven bean only when you need a JMS consumer. In other words, a bean that can listen for messages from a JMS messaging service. Clients never call message-driven beans directly; in order to get a message-driven bean to do something, a client must send a message to a messaging service. That means a message-driven bean has no EJBObject because the server gets the client requests directly from a messaging service rather than as a call from the client to the bean. A typical message-driven bean might be a NewCustomerNotification subscriber.

Session

Use a session bean for... everything else. Almost any kind of back-end service can (and often should) begin with a session bean. Where an entity bean represents a thing, a session bean typically represents a process. To put it another way, when you think of entity beans, think noun, and when you think of session beans, think verb. A shopping session is a typical example of a session bean, while a credit card processing system might be another session bean.