Three things are needed to make interactivity work in Flash: an event that triggers an action, the action triggered by the event, and the target, or object, that performs the action or is affected by the event.
Not so different than the real world, eh? Think about your alarm clock: When you set it at night, you're programming it to perform an interactive function--the underlying logic of which might look something like this:
Event. The clock reaches the time you set the alarm to go off (which sets the action into motion).
Target. The alarm clock (the object affected by the event).
Action. The alarm sounds (the action performed by the object).
If you can set your alarm, you're on your way to becoming an interactive guru.
To create interactivity with Flash, you use an ActionScript, which is simply a set of instructions that defines an event, a target, and an action.