We can define virtual objects only for objects on which we 
can click or double-click and that record a Click or 
DblClick step.

The above answer is not satisfactory Please provide 

an answer which is much clear

Virtual Object means A object that behaves like a standard
object but qtp does not identify that object.
Here we can mapping from custom classes to standard classes.

Literally "Virtual" Meaning: not physically existing as 
such but made by software to appear to do so....this is the 
exact meaning from the OXFORD Dictionary.
Here, The object "Exists" at AUT, but our QTP for some 
reasons is unable to identify, let it be a button or any 
object under test. It doesn't make any sense saying that-
 "Testing is getting retarded caz' QTP is unable to 
identify the object" but the object is there at AUT. so, in 
this scenario we are making the QTP to treat the object as 
Virtual object.

Thank you
siva.....

There's no need to ever use a virtual object. Quick Test 
Pro will find every object that you need to complete a 
transaction. Virtual Objects are useless.

VO Used for map the non-standard objects into standard objects
qtp is not identify the non-standard objects at the time we
can use virtual object wizard to map that objects into
standard objects