Is an application server something like an additional layer of application servicing above a web server?
Does an application server always have a web server as its core?
What is the difference?
No,Application server does not contain web server...
Read following articles...
http://www.answers.com/topic/application-server
http://download.oracle.com/docs/cd/E19159-01/819-3671/ablat/index.html
Basically:
After the Web exploded in the mid-1990s, application servers became Web based.
Also following shows the difference between web server, web container and application server...
Difference between a Web Server, Web Container, and an Application Server
A Web Server is a server capable of receiving HTTP requests, interpreting them, processing the corresponding HTTP Responses and sending them to the appropriate clients (Web Browsers). Example: Apache Web Server. Read more about Web Servers and their working>>
A Web Container is a J2EE compliant implementation which provides an environment for the Servlets and JSPs to run. Putting it differently we can say that a Web Container is combination of a Servlet Engine and a JSP Engine. If an HTTP Request refers to a Web Component (typically a Servlet or a JSP) then the request is forwarded to the Web Container and the result of the request is sent back to Web Server, which uses that result to prepare the HTTP Response for the particular HTTP Request. Example: Tomcat is a typical Web Container. A typical setup would be to have Apache HTTP Server as the Web Server and Tomcat as the Web Container.
An Application Server is a complete server which provides an environment for running the business components (EJBs, ADF BCs, etc.) in addition to providing the capabilities of a Web Container as well as of a Web Server. Example: Bea WebLogic, IBM WebSphere, Oracle Application Server, etc.
Actually, with the explosion of the web, and in particular "web services", all modern App Servers can also function as Web Servers. For example, the current version of Tomcat includes built-in Web Server functionality so you no longer must run a separate Apache HTTP server. In the past, running separate dedicated Web Servers such as Apache was preferable since the App Server was optimized for App Server rather than for Web Server, processing; but the performance of App Servers as Web Servers has improved such that any remaining performance difference is irrelevant - and certainly does not justify the expense of running separate servers.
Today, the major difference is that an App Server is designed to support programming languages such as Java or, on the .net platform, C# - as well as to provide an underlying infrastructure that includes automatic fault-tolerance, session mgmt, transaction mgmt, multi-threading - and everything else required to build scalable enterprise applications. Current Web Servers are designed to support languages such as Ruby, PHP, Python and Perl - and lack the built-in infrastructure of an App Server.
However, the distinction between App Servers and Web Servers is blurring and will continue to do so as "Web Services" becomes ever-more popular and languages such as Ruby mature and therefore require Web Servers to provide much of the same underlying infrastructure as today's App Servers. In the end, the primary difference will be (not yet): if you want to develop the back-end (cloud) layer of your application employing Java or C#, employ an App Server; if you want to develop your application employing Ruby, PHP or Perl, employ a Web Server.
While a Web server mainly deals with sending HTML for display in a Web browser, an application server provides access to business logic for use by client application programs.
来源：https://stackoverflow.com/questions/5481902/web-server-vs-app-server