By using 'Define new test object' we can create a new test 
object in object repository, even the application is not 
available.

You are exactly right...But
What is the use of Creating new test object, if it not work 
on the application.what will we do with it.How to use on 
the application????
Please tell me with one example

yeah good question...

   in some situations we have to prepare the scripts before 
the application is delivered for testing. at that time you 
can not capture the objects from application, right? so, 
what you can do is you can create the test objects manually 
based on the test case says, and can prepare the scripts. 
once the application is ready we will debug the script to 
check whether QTP is identifying the objects with the user 
defined test object description. if not identifying u can 
change the properties of those objects.

   and another situation, when you are capturing some 
objects to Object Repository some of the objects may be 
invisible. they will come to visible only in some 
situations. so, you will create one user defined test 
object even that object is invisible. 

    may be this can answer your question, if not please 
reply me again. i can add some more description.

Thank you nanda...
U r right.. 
Second one is Okay..
But one thing I have not understood is that, after writting 
the code, should we add properties to the repository?
I knew two ways to write script.
1. Add objects to the repository and use them in script.
2. Assigning the property values to the properties 
directly, without using OR (Discriptive programming)
But here not using either OR or object property values in 
the script
How the QTP identfies the objects of an application.
suppose I have written like this.(if it is web based)

Browser("Rayudu").Page("ray").link("livetek").click

Once application ready how can I run this on the 
application.

ok
should we use settoproperty???