What the different between GUI Map and Object Repository

Hi,
I am confused. I am new to QTP.
A test will not be runningn without GUI Map within WinnRunner which means GUI Map must be exist before recording.

In QTP, I can start recording ... insert logic... then run.
Could you explain why?

What the different between GUI Map and Object Repository? Let's say I record as page, I can I use the recorded repository on a new test? How?

Thanks

GUI Map: WinRunner stores information it learns about a window or object
in a GUI Map. When WinRunner runs a test, it uses the GUI map to locate
objects. It reads an object’s description in the GUI map and then looks
for an object with the same properties in the application being tested.
Each of these objects in the GUI Map file will be having a logical name
and a physical description.

Object Repository: It is an advanced feature availale in QTP, an object repository is build to add all the objects that you want to test in your application or Web site.
QuickTest identifies objects in your application based on a set of test object
properties. It stores the object data it learns in the object repository.
When you run a test or component, QuickTest identifies each object in your
application by its test object class and its description—the set of test object
properties and values used to uniquely identify the object. The list of test
objects and their properties and values are stored in the object repository.
You can save your objects either in a shared object repository or in action object
repository. In shared object repository mode, you can use one object
repository file for multiple tests or components. In object repository peraction
mode, QuickTest automatically creates an object repository file for
each action in your test. Object repository per-action mode is not available
for components.

the detailed information on OR can be found in QTP's user guide

Thank for the explaination.

How do I create an object file to be shared among tests?