An event may be defined, very simply, as an action performed on a Web page - for example, clicking a button, moving the mouse pointer over a hyperlink and so on. Events are not necessarily generated by the user; they may be generated automatically by a piece of code as well.
An event handler, as the name suggests, handles an event - it defines the action to be taken by a script when a particular event (or type of event) occurs. Event handlers exist for most of the common events that are generated on a Web page, including mouse movement, mouse clicks, keyboard activity and page loads.
Here's a quick example that might make this theory a little clearer:
Another example, and one you've probably seen before, is