Session beans are a key technology within the J2EE platform because they allow business functionality to be developed and then deployed independently of the presentational layer.
For example, you might create an application with a user interface built using Java's Swing API. This application might then provide access to some business functionality for the employees working on the company's internal network.
If the underlying business functionality is implemented as Session beans, a different user interface could take its place without having to redevelop the entire application. A Web-based interface would make the application available from the Internet at a single stroke.
There are two types of Session beans, and a couple of analogies ...