Dear Narendra,
What Kedar has written is right.
(1) Basically web.xml is a deployment descriptor for application.
When you have servlets in application you require
1.1) web.xml : Using web.xml developers can
declare initialization parameters for Servlets (ServletConfig)
and initialization parameters for application (ServletContext).
Not only this but some times we also require init parameters for
jsp. So they can be given to jsp using web.xml
We can also configure session timeouts, listerners etc. So
web.xml(deployment descriptor) is required
1.2) WEB-INF -> classes : In this directory we store servlets,
java beans, utility classes. So this also required.
1.3) WEB-INF -> lib : Some times we use third party library i.e.
jar files in our application. That time we keep these lib files
in lib directory. If you dont want to use this directory you can
keep library i.e. jar file in common/lib folder which is under
Tomcat Home Directory.
So if you are using only jsp based application, then you might be having few java beans used with it. So these all directories are required.
~Amit