Permissions control the level of a user's access to the Jira Customer Service Management instance, while roles are how the permissions are assigned to individual users.
Global - These apply to applications as a whole, not individual projects.
Project - Organized into permission schemes, these apply to projects.
Issue - Organized into security schemes, these allow the visibility of individual issues to be adjusted.
Roles
Jira System Administrator - can perform all Jira administration functions.
Jira Administrator - can perform most Jira administration functions.
Service project Administrator (Project role - Administrators) - assigned to specific service projects and manages those service project's configurations.
Agent (Project role - Service Desk Team, Project role - Customer Service Team) - assigned to specific service projects and manages and responds to Requests.
Customer - can submit and update their Requests, and may participate in Requests raised by other Customers.