In general, roles represent a collection of users that are responsible for similiar areas, have the same status or perform the same tasks in an organization.
Cognos 8 allows administrators to implement custom business specific roles, however a default set of ten predefined roles will normally be enough.

The IBM Cognos 8 BI predefined roles are:
• Consumers - run and read reports
• Query users - run and read reports and use Query Studio
• Authors - run and read reports and use Query Studio and Report Studio
• Report Administrators - administer reports, use Query Studio and Report Studio
• Server Administrators - administer servers, jobs and dispatchers
• Directory Administrators - administer accounts, groups, contacts, distribution lists, data sources and printers
• Metrics Administrators - administer metrics packages and tasks in cognos connection
• Metrics authors - Create and edit scorecard applications in Metrics Studio
• Metrics users - monitor enterprise performance in Metrics Studio
• Portal administrators - Administer cognos portlets and third-party portlets in Cognos Connection