When you insert a call to a copy of an action into a test, 
the original action is copied in its entirety, including 
checkpoints, parameterization, the corresponding action tab 
in the Data Table, plus any defined action parameters. If 
the test you are copying uses per-action repository mode, 
the copied action's object repository is also copied 
together with the action.
You can insert a call to a reusable action that is stored 
in your current test (local action), or in any other test 
(external action). Inserting a call to an existing action 
is like linking to it. You can view the steps of the action 
in the action view, but you cannot modify them.

call to action : the changes made in call to action, will be
reflected in original action(from where the script is
called).But where as in call to copy Action , the changes
made in the script will not effect the original
script(Action).call an existing action means calling a
reusable action that is stored in same test or in some other
test wich is like a function call.

i think there is only 
1.call to new action -- creates new action
2.call to copy of action -- copies existing action and we 
can modify that and the original one will not change
3.call to existing action -- calls existing action action 
and we can't change the script, in the called action

and there is no "call to action" and we can chage the 
script and we can change the original one too.

let me know if i am wrong.

Sowmya,

You are right. We should use the Call to Copy of action 
only when we need the change the code and if there is no 
change needed we can use Call to existing action and we can 
edit the data which is coming from the external action. I 
prepared a detailed document with questions and also 
answers and it is available on my website 
http://www.questionsforinterview.com which will answer all 
such questions.

Sridhar Mallepally

call to action:
any action can be called with in the text and with in the 
another text.we can call action by default qtp creates copy 
of action

call to existing action:
it is reusable action with in the text or with in the 
another text.qtp creates a link to called action and make 
to statements as read only calling actions.

if it is wrong plz tell the correct.............

                                 thank you

sowmya's answer is perfect than others.

I think "Copy of Action" or "Call to Action" is the same
thing, if a user wants to call an external action, he/she
makes a copy of that to use it in QTP, in these terms it is
called "Call to Action".