The studios in Cognos 8 are simply user interfaces to allow reports to be defined. Queries are generated at runtime based on both the underlying package definitions and the grouping/calculations/filters etc defined within the report.
Query Studio is targeted at Business Authors who have little or no technical experience, and consequently the UI is task-oriented, simple to use, and avoids use of complex expression editors. Report Studio is for professional authors (power users) with much better technical skills, and allows use of expressions, manipulation of query items, multi-query reports etc.
Regards,
MF.