How to utilize max use of DEBUG?

HI Expert's, Guru's and Sifu's

As an abaper, i would like explore more on Debug. My questions are:-

1) What is the use of "desktop 3" ?

2) The use of break and watch point?

3) the use of Abap Stack?

4) the function of Variable 1, Variable 2, Locals and Globals?

5) When i double Click on the Tabels, by default the information will be in Variable 1, but how to make USE this Interface?

That's all for now....

please Guide me ...

Thanks, Regrs

mayat