Cheat Sheet:
   http://hotwired.lycos.com/webmonkey/reference/html_cheatsheet/

<a href="http://www.schmoop.com/" 
onMouseOver="document.img.src='images/rolloverimg.gif';"
onMouseOut="document.img.src='images/regularimg.gif';"> 
<img src="images/regularimg.gif" name="img"></a>

<form>
<input type="button" value="Click me please" onClick="alert('Mmm, 
that feels good.');">
</form>

As you can probably deduce, the active ingredient here is "onClick" 
-- a simple handler that refers to the event when a button or link 
is clicked. When that event happens, the JavaScript commands following 
the onClick are executed. If you need an elaborate action to take place 
when the button is clicked, you can call a predefined function at that 
point. All of the event handlers work pretty much the same way, and vary 
only in what target elements are available: a button is susceptible 
to being clicked, whereas a form is not. Here is a rundown of the common 
event handlers, ones that both Netscape and IE can deal with. (There 
are a bunch more that only work in one browser or the other, but those 
are a pain.)

In alphabetical order:

onAbort is triggered when an image is stopped from loading because 
the user either hits Stop or leaves the page.

onBlur is triggered when an element, such as a window, frame, or form 
field, loses focus; that is, when the user clicks on something else.

onClick you already know.

onChange is triggered when the value of a form field changes, for 
example, when the user types in some data.

onDblClick is triggered by a double-click.

onError is triggered by a loading error, like a missing image.

onFocus is the opposite of onBlur: it's triggered when the user puts 
the focus on the target element, by clicking on it or tabbing to it.

onKeyDown is triggered when a key on the keyboard is pushed down, 
regardless of whether it is then held down or released.

onKeyPress is repeatedly triggered as long as a key is held down.

onKeyUp is triggered when a key is released.

onLoad is triggered when the browser completely loads the page.

onMouseDown is like onKeyDown but for the mouse button.

onMouseMove is triggered when -- yes -- when the mouse moves.

onMouseOut is triggered when the pointer moves out of the target 
area (off of an image, for example).

onMouseOver is the opposite of onMouseOut: it's triggered when the 
pointer moves over the target element.

onMouseUp is like onKeyUp but for mice.

onReset is triggered when the Reset button of a form is clicked.

onResize is triggered when a window or frame is resized by the user.

onSelect is triggered when the user highlights text in a form field.

onSubmit is triggered when a form is submitted.

onUnload is triggered when the user leaves the page