An event is the representation of some asynchronous occurrence (such as a mouse click on the presentation of the element, or an arithmetical error in the value of an attribute of the element, or any of unthinkably many other possibilities) that gets associated with an element (targeted at it) in an XML document.
In the DOM model of events [DOM2EVENTS], the general behavior is that when an event occurs it is dispatched by passing it down the document tree in a phase called capture to the element where the event occurred (called its target), where it then may be passed back up the tree again in the phase called bubbling. In general an event can be responded to at any element in the path (an observer) in either phase by causing an action, and/or by stopping the event, and/or by cancelling the default action for the event.