Row Group by and when to use "Add Group Header"

Hi All,

I am just starting to use SSRS R2 and I have a question about Groups.

When I create a Tablix group, the first thing I do is go to Row Groups, Add Group, Parent Group select the correct element to group by; no problem here.

The problem/confusion comes about is when do I want to select "Add group header" and/or "Add group footer" or select neither?

Will someone please explain when to use / not use these options when grouping? What circumstances / examples can you refer me to so I can visually see the outcome of choosing or not choosing these options.

Thanks for your help.

Dave