A Business Role can be a person or machine. One person can be member of multiple business roles (e.g. Sales manager and Employee), and multiple persons can be member of the same business role.
The business role is the one that launches a process, makes decisions on which of the available paths in the process to take and in the end finishes the process.
In an SAP context a (technical) role is also defined
Roles are collections of activities which allow a user to use one or more business scenarios of an organization. The transactions, reports and web-based applications in the roles are accessed via user menus. User menus should only contain the typical functions in the daily work of a particular user.
See also help.sap.com