> what is design patterns.
"Design patterns" is a literal phrase, referring to structures of code,
process or analysis that recur in many situations. These patterns are useful
in system design.
These patterns go by conventional names, though perhaps you have seen them
yourself and had a different term for the same idea. One well-known (and
oft-cited) pattern is the "Visitor" pattern:
The idea is to allow an unbounded set of actions on an object by having action
objects "visit" the accepting object. The accepting object accepts a Visitor
and uses the visitor.visit() method to do something. Visitor implementations
can do just about anything.
That description is a bit vague because it only describes the pattern, not how
you'd actually do it. For example, the interfaces might or might not be named
Visitor, Acceptor, etc., or the methods visit(), accept(), etc. The Wikipedia
article outlines several different fundamental approaches, such as functors.
> How to use them .
Keep them in mind, and use them to organize your design thinking. Use the
pattern to verify the correctness of an implementation. Know many different
patterns because some work better than others in different situations, and you
can combine them for powerful results.
Beware of antipatterns: patterns that are often employed but usually harmful.
> Could anybody plz [sic] tell me how to aproch to design a web application
> with proper architecture
Start with this excellent advice:
The masters say, Always keep beginner's mind.
Search Wikipedia, Sun and Google (the "G" in "GIYF") for the JEE tutorial, and
check out the Model-View-Controller (MVC) architecture, a.k.a. the "Front
Controller Pattern", and its variants.
Apache's Tomcat site has excellent documentation on web-app development,
including recommended layouts for source and deployment directories, some of
which are mandatory.
Please spell "please" as "please", if you please.