Event : It is an action that is performed by the user, while interacting with an app. In Lightning component, event are fired by the JavaScript controller actions. It is declared by the aura:event tag.
Type of Event :
Application Event : It is used when you need two independent component to communicate with each other.
Component Event : It is fired from an instance of a component. It can be handled by the component that fired the event or by a component in the containment hierarchy that receives the event.