Who is Actor in UseCase diagram ?

How to identify the Actor in Use Case diagram ?

   Ask following question to find the actors :

• Who use the system ?

• Who install the system ?

• Who Starts up the system ?

• What other systems use this system ?

• Who gets the information from this system ?

• Who provides information to the system. 

Actor is always external to the system. They are never part of the system to be developed.

Understand by simple project namely Online campus selection system.
Answer of above question are : 

• Student

• Company

• Admin (here College)

These are the Actor of use case diagram.