An event is a notification that something has occurred (such as a mouse click) or, in some cases, is about to occur (such as a price change).
Classes can define events and their instances (objects) may raise these events. For instance, a Button may contain a Click event that gets raised when a user has clicked it.
Event handlers are then methods that get called when their corresponding event is raised. A form may contain a Clicked event handler for every Button it contains, for instance.