Basic difference between Application server & Webserver

I want to know the basic difference between Application server(Like Websphere) and the Webserver(Apache Tomcat) server.

can any please tell me in detail why we need Applcation server and Webserver.

first: Apache Tomcat is not just a webserver. Many people would in fact call it an application server.

Websphere, iPlanet, SunONE, JBoss -> These things are usually called app servers, and what they really mean is "it does EJB's".

Tomcat is not quite a "full-blown" app server, because it only does servlets/jsps and not EJBs.

a plain old "web server" (like Apache), serves only HTML pages (as long as it doesn't have any modules).