Dear Samina
Report Painter is basically a tool that gives you option to create FI & CO specific reports. It can be specific to AR, AP, GL, CO-IO, CO-CCA,,,etc.
Answer to your First & second question is that GRR reports are majorly concentrating on CO reports and very less of FI reports and they are based on specific tables created for GRR reports. Where as FGI/FSI reports majorly focuses on AP/AR/GL only.
Another major difference is that, in GRR reports you only create forms and you are ready with your reports. You are limited to the characteristics available in your library. Where as in FGI/FSI reports, you have an option to play on more characteristics available not only at Form level but at report level too.
Frankly speaking, theoretically it is difficult to explain the difference unless you actually work on them.
Let me know if you need more inputs?
Best Regards
Syed Zia Abbas