Every object on your page has certain 'Events' which can trigger your JavaScript functions. For example, we use the 'onClick' event of the form button to indicate that a function will run when the user clicks that object. We define the events in the same HTML tag which we use to place an object on the screen. So a button which runs 'myFunction()' when clicked will be written this way.

<input type=button value="click me" onClick="myFunction()">

I could also use onMouseOver to make an event happen when the user's mouse is over certain objects. For instance a message could appear in an alert box.

Images can't have events, but links can. So I added an onMouseOver event to the above image by creating a link to nothing around it. Here is the code for the above image:

<A HREF="#" OnMouseOver="alert('hi')">
<IMG SRC="http://images.webteacher.com/javascript/images/pointme.gif" BORDER=0 >
</A>

The following table outlines all of the event handlers in NetScape version 3.0
(SOURCE: NETSCAPE'S JAVASCRIPT REFERENCE PAGE)

Event	Applies to	Occurs when	Event handler
abort	images	User aborts the loading of an image (for example by clicking a link or clicking the Stop button)	onAbort
blur	windows, frames, and all form elements	User removes input focus from window, frame, or form element	onBlur
click	buttons, radio buttons, checkboxes, submit buttons, reset buttons, links	User clicks form element or link	onClick
change	text fields, textareas, select lists	User changes value of element	onChange
error	images, windows	The loading of a document or image causes an error	onError
focus	windows, frames, and all form elements	User gives input focus to window, frame, or form element	onFocus
load	document body	User loads the page in the Navigator	onLoad
mouseout	areas, links	User moves mouse pointer out of an area (client-side image map) or link	onMouseout
mouseover	links	User moves mouse pointer over a link	onMouse- Over
reset	forms	User resets a form (clicks a Reset button)	onReset
select	text fields, textareas	User selects form element's input field	onSelect
submit	submit button	User submits a form	onSubmit
unload	document body	User exits the page	onUnload

More about Events