AOM: Automated Object Model with this model we can create 
QTP object.  without opening QTP, we can do open Test, add 
repositories, recovery scenarios, .vbs files, Datatable
(excels) etc.

DOM: Document Object Model this is come under HTML like 
that. i have n't worked on this.

COM: Component Object Model with this model we can create 
Excel, MS-Word, OutLook objects etc.
with this object we can do with out opening Excel 
application, we can add sheet and count the rows and 
columns etc.

like this way we can use these objects based on criteria