cookie vs. database

Hi, In a typical E-commerce application, e.g. an online bookstore, I have all books listed and a button "Add to cart" next to it so user can just click on that to buy it. Question is: I found on a book the authors use cookies to store those information related to books clicked by user, instead of inserting them to the backend database table "transaction"(although it is created and already there). Is there any reason we need to do it this way, or just 2 options? I am thinking it might have sth. to do with the stateless feature of HTTP request, but still can't see exactly why. Any help is most appreciated. hxl ________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com