Define the different types of objects that are used in QTP (quick test pro)?

There are several object types that are very commonly used in QTP:

Web objects:
Standard Windows objects:
Visual basic objects:
Utility objects:
ActiveX objects:
Supplemental Objects:

All these objects methods and properties are used to test the respective object model.
For example, visual basic objects methods and properties are used to test visual basic objects.

What kind of applications (software technologies) we can test efficiently using mercury QTP (quick test professional) ?

QTP stands for quick test professional which is a mercury’s product for automation testing (functional & regression testing) of below technologies.

Windows applications,
People soft
Java
SAP
ERP
Oracle
Siebel
VB
Dot net
ASP
Java script
And some Others…

Note: you need add-ins for using QTP on different technologies.
For example to work on ERP(SAP, people soft etc..) applications you need SAP add-ins, people soft add-ins etc…