Typically a web server just serves up web content and, sometimes, web applications such as those built using JSP, Servlets, ASP, etc. Application servers are generally used for building distributed, component based transactional systems with technologies such as EJB, CORBA, .NET, etc. That's the basics anyway... Simon
There is a logical difference between them as per specifications. In a typical 3 tier architecture, usually server ( Webserver + Appserver) sits in the middle tier. webserver can be identified as the one talking to first tier (usually Client) , it deals with clients directly and presentations, which serves pages like JSP, ASP or HTML. Appserver is the one which takes the request from the Webserver and returns the response might be by accomplishing this by talking to the third tier (usually DataBase) There are compact servers with both webserver and appserver together are available in the market. Ex: BEA's weblogic, Sybase's EAServer etc.