i have one question, how can i link the web server to an application server and link the application server to the database server. and can somebody tell me how effective this kind of combination will be over using a application server with a database server. if possible the pro's and con's.
Tomcat has a Connector for hooking it up to Apache (or IIS). The details are linked from that page. Whether or not a web server/app server combination (like Apache httpd + Tomcat) makes more sense than just a standalone Tomcat is impossible to say in general, as it depends so much on your particular needs.
A long time ago people would use a web server alongside Tomcat because Tomcat's performance at serving static files sucked. That's no longer the case, and Tomcat will serve static files with quite good performance.
Using httpd or IIS is indicated if they provide features you need that Tomcat simply doesn't have, e.g. ASP support in IIS, or some Apache module like mod_python in the case of httpd. httpd can also provide useful other features like SSL termination and load balancing for a cluster of Tomcats.
As to databases, the "link" generally consists of a JDBC connection over TCP/IP. Any JDBC tutorial will tell you how to use that.