Hi sheel,
- ABAP classes - most used for every oo purpose
- Exception Class - to be used for raising an exception object, mostly used in CATCH section of TRY..ENDTRY statement
- Persistent class - a class that will keep data after leaving (closing) the current logical unit of work (LUW, transaction), involves a lot of overhead, not very frequently used, nothing for beginners
- Test Class - used in automated testing. Meanshile also SAP provides a test framework as i.r. JAVA does since long. Customers using ABAP unit tests with test classes and test cockpit are still rare (I don't know any)
Regards,
Clemens