An event (org.zkoss.zk.ui.event.Event) is used to notify application what happens. Each type of event is represented by a distinct class. For example, org.zkoss.zk.ui.event.MouseEvent denotes a mouse activity, such as clicking.

To response to an event, an application must register one or more event listeners to it. There are two ways to register an event listener. One is by specifying the onXxx attribute in the markup language. The other is by calling the addEventListener method for the component or the page you want to listen.

In addition to event triggered by user's activity at the browser, an application could fire events by use of the sendEvent, postEvent and echoEvent methods from the org.zkoss.zk.ui.event.Events class.