Diff Btwn Webserver and Application server

What is the Main Difference Between Webserver (like Tomcat etc..) and
Application Server (like Weblogic and Websphere)..

cant we able to run 3-tier architecture applications using the Web Servers

Weblogic(application server) contains a webserver. Most of the application servers contains webserver. Tomcat is a appserver.

A Web Server understands and supports only HTTP protocol whereas an Application Server supports HTTP,TCP/IP and many more protocols. Also many more features such as Caches,Clusters,Load Balancing are there in Application Servers which are not available in Web Servers. We can also Configure Application Servers to work as Web Server. In short, Applicaion Server is a super set of which Web Server is a sub set.
[ March 01, 2005: Message edited by: Sunitha Raghu ]

Tomcat is not a webserver in true sense, it is a servlet container and hence the capability of serving JSP/Servlets. Apache by itself is a webserver (it can serve html and static stuff ) but cannot serve jsp and servlets.

Tomcat is a appserver.

Why is Tomcat a app server ?