Application Server Vs Web Server

I have been bugged with this question many a times so finally i have decided to write a small blog entry on the same.
An Application Server (like Glassfish) is more sophisticated and complex (read intelligent) when compared with a Web Server (like Tomcat). A Web server is based on HTTP request-response model and generally acts as a web container for JSP's and Servlets. You give it a request and back comes the reply.
On the other hand, an Application Server can be used to serve business logic to applications programs, generally in a n-tier architecture (n>2), through any number of protocols (HTTP, HTTPS, IIOS/SSL). It is this capability of an appserver to cater the needs of a separate business layer in a 3-tier architecture through a component API (like EJB's) that makes enterprise level applications far more scalable. By separating the business logic from presentation layer, we make the business logic reusable between and within applications. Moreover, an application server manages its own resources. It takes care of other important issues like Transaction Management, Security, Database Connection Pooling, Clustering, Scalability and Messaging etc. A web server cannot provide these.
An Application server like Glassfish also provides the administrator with something known as Glassfish Admin Console, using which he/she can easily manage and utilize various resources like Connection Pools, JavaMail Sessions. More on this coming up soon.
Generally, all application servers contain a web server in them or you can say that a web server is a small subset of what comprises of an Application Server. Generally, all application server comes with two types of containers:
• Web Container
• EJB(Enterprise JavaBeans) Container
Difference b/w Glassfish & Tomcat in Arun Gupta's words,
- Tomcat is only a JSP/Servlet container. Everything else such as Web services, all the "Web 2.0" style processing, etc need to be installed in the container. Because it's only JSP/Servlet container, it's light-weight.
- Glassfish is a full Java EE 5 compliant App server. JSP/Servlet is just one component of Java EE 5, then there is Enterprise Java Beans, Web services, XML Binding, Security, Reliability, Transactions, Clustering, High Availability, Fault Tolerance and such enterprise features. GlassFish comes pre-bundled and pre-configured to handled those. In case of Tomcat, you need to install additional software for each of these components.
As a matter of fact, i'm an avid follower of Glassfish and strongly recommend Glassfish to developers worldwide, various reasons pamper me to do so:
• It is Open Source
• It's supported, maintained & developed by Sun Microsystems and others too.
• It is fully Java EE 5 compliant and any new technology as and when introduced (even in future), GF will support that also.