An actor specifies any role played by a user or any other system that interact with the system including system components only if they responsible for triggering a service or use case. Actor can also represent another system in which the present system interacts. Basically, actors represents the principal elements in the environment in which the system operates.
Primary actor is the stakeholder who uses system services to achieve the goal. It always has a goal with respect to the system and the use case exists to satisfy the primary actor. Most of the times, primary actor who triggers the use case.
Supporting actor is an external actor provides a service to the system under design. System needs assistance from supporting actor to achieve the goal of primary actor. Secondary actor might have goals that they expect to be satisfied by the use case.
Offstage actor is the stakeholder who has interest in outcome result that must be satisfied, but who has no active role in use case.
Actor has goals to achieve and has to take an actions. Achieving a goal might requires the completion of sub-goals. Secondary actors come into view to support the system in providing resources to achieve these sub-goals. Calling actor must wait for the secondary actor to achieve the associated sub-goals.