if the exaple is not clear
here is the example
www.chase.com---> Static html pages which are stored on webserver example Apache/Iplanet so on
once you make any request after login getting your balance any details will be served by app servers like weblogic/websphere
static page is same for all usrer and will change once you login with ur credentials
Simple saying place where entire Business logic exists in application server in pulling required information from DB
Request---> Webserver ---> Application Server ---> DB
<-- <-- <--- responce
Let me know any more Q's more then happy to share info and learn.