Application Server:
Application server maintains the application logic and
serves the web pages in response to user request.
That means application server can do both application logic
maintanence and web page serving.
Web Server:
Web server just serves the web pages and it cannot enforce
any application logic.
Final conclusion is: Application server also contains the
web server.
For further Reference :
http://www.javaworld.com/javaqa/2002-08/01-qa-0823-appvswebserver.html