Two primary techniques to identify use cases are USER GOAL TECHNIQUE AND EVENT DECOMPOSITION.
Use case refers to an activity which a system perform as a result of the request input by a user.
User goal technique is used to determine the specific goal that must be perform by the system for the user; user goal technique identify all the users of the system and their goals for using the system.
The event decomposition is used to determine the event to which the system must respond.