Event

In computing, an event is an action or occurrence recognized by software that may be handled by the software. Computer events can be generated or triggered by the system, by the user or in other ways. Typically, events are handled synchronously with the program flow, that is, the software may have one or more dedicated places where events are handled, frequently an event loop. A source of events includes the user, who may interact with the software by way of, for example, keystroke on the keyboard. Another source is a hardware device such as a timer. Software can be trigger its own set of events into the event loop, e.g. to communicate the completion of a task. Software that changes its behavior in response to events is said to be event-driven, often with the goal of being inactive.

Event handler

In computer programming, an event handler is a callback subroutine that handles inputs received in a program(called a listener in Java and JavaScript). Each event is a piece of application-level information from the underlying framework, typically the GUI Toolkit. GUI events including key presses, mouse movement, action selections, and timer expiring.