Here is the difference
* Servers Static contents
* Directly receives clients request
* Deligates with tomcat container or application sever to process the business logic
* Finally sends the response to the client.
* For processing the business logic
* Interacting with database and legacy systems
* Application server does not interact with the client browser.
* Application server is design to process business logic
For more information please check: http://www.roseindia.net/web-application-server/tomcat/web-server-and-application-server.shtml