When using a Report Layout/Organization type of Report you can have a field type of Subreport. Including a subreport within another report causes the subreport to be executed once for each row in the result set of the parent report.
Subreports can be used to create a "parent/child" type subreport. For example summarize open issues by project (parent) and then show top ten oldest for each project (child).
Subreports can also be used to create a "dashboard" type effect.
