Oracle and NetDynamics don't have entity beans, and I think that is a problem, because they are two of the biggest players in this space. But it is important to use entity beans, because they are a critical part of the application design. The session bean is used to represent the stateful dialogue between a server and a client, such as calculating the amount of money deposited into a bank account. The entity bean represents an actual entity, such as a bank account, and its associated rules. Accordingly, entity beans map into the session and the underlying data store.
Entity beans are always persistent.
If a user leaves the bank site and comes back a day or even a year later, he or she will still be able to access the underlying data represented by the bank-account entity bean. With stateless session beans, any state required is maintained on the client to minimize the impact on server resources. With stateful session beans, state data can be stored on the server, although in the event of a server crash the state is lost. Entity beans further simplify the transition to three-tier computing by providing a higher level of abstraction. Since entity beans represent an underlying business entity, they map well into application modeling principles and practices that are commonly used in the enterprise today.