LOCAL_C, LOCAL_D, GLDEF_C please help

Dear all,
These problems i faced while creating Symbian Console based applications for S60 platform.'

1) Can you please tell me the difference between LOCAL_C, LOCAL_D and GLDEF_C?

2) Whenever i write any class of my own, should i start it always from MainL() or from E32Main()?

3) When i create any class of my own (not any inbuilt class) is it mandatory to always include the
class template in the .h (header file) and the definition in .cpp file? Or does it violate the coding
standards, if i include the whole class definition in the .cpp itself?

4) Once i tried to call a class (create object) from MainL ( ) function whose prototype i gave in a
separate header file. It gave an error message. The error was rectified when i mentioned
MainL( ) as GLDEF_C, and all other functions which calls MainL( ) as also GLDEF_C.
Can you please explain me why is it so?

Please help me, i am a beginner to Symbian and need all your help.

Thanking You,
Raj Kumar Sanpui