An entity bean represents persistent global data from the database; a session bean represents transient user-specific data that will die when the user disconnects (ends his session).
Generally, the session beans implement business methods (e.g. Bank.transferFunds) that call entity beans (e.g. Account.deposit, Account.withdraw).
Session beans can act as agents modeling workflow or provide access to special transient business services. As an agent, a stateful session bean might represent a customer's session at an online shopping site. As a transitive service, a stateless session bean might provide access to validate.
Session beans do not normally represent persistent business concepts.