object repository works as the interface between QTP and 
the application..whenever we record the properties of the 
objects are stored in object repository and when u replay 
the script QTP compares the the objects in the application 
with the recorded description that is being stored in 
object repository..

object repositiry is nothing but interface between tool and
application build.actually during recording qtp generate the
script and at the same time it add all the object properties
add to the object repositiry.during running it catch all the
object properties from object repository,based on that
propertis it tdentify the object.two types of object
repository are there.
1-per action:this is the type of object repository.here qtp
maintains single file for each action.extension of peraction
repository is .mtr(mercury test reposirory) 
2-shared repository:here qtp maintains single file for each
action.extension of shared repository is .tsr(test shared
repository)

Object repositiry is the heart of the tool in which the 
references of the objects are stored.without object 
repository tool unable to identify the object.In one 
line,properties of the objects are available in the object 
repository.

We r using object repositiry to itroduce the appliction or 
software to QTP.Wtihout object repositiry we can't idetify 
the property of oject.After intoducing we can execute the 
application in QTP