Start End Routine Sample Code with Explanation

Hi All,

Can anyone explain me How to write code  with the given Example

in START and END Routine

                                      TRANSFORMATION

Data Source  Fields                                                             Cube Fields

       A  ------------------------------------------------------------------->   A + B

       B  ------------------------------------------------------------------->   B  * A

       C  ------------------------------------------------------------------->   C

       D  ------------------------------------------------------------------->   D

Please explain the statements with its significance like LOOP AT,Internal Table concept wit work area,TYPES,DATA etc

Thanks in Advance,

Mayur