Hi,
Session beans, as the name suggest are used to execute a business transaction while the client session is alive. During this time entity bean could be used to represent the data manipulated by the sesion bean.
For e.g. adding a book to your shopping cart.
Session Bean - add a book to shopping cart.
Entity Bean - A book.
for more information about difference between both the bean types ... read.. http://forum.java.sun.com/emailmessage!default.jspa?messageID=9502230