Question

Difference between the data source and the main document ??

Answer 1

The main document is usually the presentation tool, how the data is displayed to the user. The data source is where the data comes from. 

For example, excel and crystal reports can present the data based on embedded queries back to an SQL or Access database (the data source). 

The main document can use graphic items (charts, tables, etc.) that wouldn't be in the database itself. It can collate the data into a meaningful form for the reader.