Explain the concept of object repository & how QTP recognizes objects?

Object Repository: displays a tree of all objects in the current component or in the current action or entire test (depending on the object repository mode you selected). We can view or modify the test object description of any test object in the repository or to add new objects to the repository. Quicktest learns the default property values and determines in which test object class it fits. If it is not enough it adds assistive properties, one by one to the description until it has compiled the unique description. If no assistive properties are available, then it adds a special Ordinal identifier such as objects location on the page or in the source code.