Required a knowledge on the Developing Complex Reports..!!!

Hi All,

 Question: Required a knowledge on the Developing Complex Reports..!

 I am Having Good knowledge on the Sql/plsql side also developed few reports. But comes to the developing or understanding the existing report giving some confusion.

i.e. Reports are using  number of Groupings ,this is really confusing . How to understand this kind of reports in a simple manner.

( It would really  helpful to me ,if you provide a complex report (Having min of 5 groups) and explaining. Also ,Is there any User Guide for Developing reports ?.

  Please provide your valueble and helpful ideas to improve my knowledge and to move forward.