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• Site administrator - the Moodle administrator role.  Typically held by IT personnel
• Manager - a site-wide role that can manage individual courses but not change the Moodle system itself.  Typically held by Help Desk, Library, or other support personnel.
• Course Creator - a site-wide role that can create individual courses.  Not used much at Swarthmore
• Authenticated User  - a role that any logged in user has
• Web Service Account - a role given to automated systems that interact with Moodle.  For example, the Moodle Mobile app may use the Web Service Account to send and receive data from Moodle.  

More about roles

It is possible to have multiple roles within a single course.  Moodle has a complex system for determining what permissions users have when given multiple roles, but generally the permissions are additive.  The most common case may be that a student is manually enrolled in a course and then is enrolled through the Registrar's Office.  The student will end up with both the "Registered Student" and "Student" role.  Since these roles have the same permissions, everything will work fine.  

You can find more details on the Roles and Permissions page of the Moodle documentation site.