Session bean is for business logic where as entity bean is used for data. You can represent credit card as a entity bean, where as you can use a session bean for a convertor.
From Ed Roman mastering EJB2.0 page 62
Session beans. Session beans model business processes. they are like verbs because they are actions. The action could be anything, such as adding numbers, accessing a database, calling a legacy systems, or calling other enterprise beans. Examples include a pricing engine, a workflow engine, a catalog engine, a credut card authorizer, or a stock trading engine.
Entity beans. Entity beans model business data. They are like nouns because they are data objects-that is, java objects that cache database information. Examples include a product, an order, an employee, a credit card, or a stock. Session beans typically harness entity beans to achieve business goals such as a stock-trading engine(session bean) that deals with stocks 9entity beans).