A Use Case is a methodology used in system analysis to identify, clarify, and organize system requirements. The use case is made up of a set of possible sequences of interactions between systems and users in a particular environment and related to a particular goal. A use case can be thought of as a collection of possible scenarios related to a particular goal.
Actor we mean as user, playing a role with respect to the system, seeking to use the system to accomplish something worthwhile within a particular context. Actors are generally people although other systems may also be a actors.
Scenario is sequence of steps that describes the interactions between the actor and the system.