In JavaScript:
pageX, pageY, screenX, screenY, clientX, and clientY returns a number which indicates the number of physical “CSS pixels” a point is from the reference point. The event point is where the user clicked, the reference point is a point in the upper left. These properties return the horizontal and vertical distance from that reference point.
pageX and pageY:
Relative to the top left of the fully rendered content area in the browser. This reference point is below the URL bar and back button in the upper left. This point could be anywhere in the browser window and can actually change location if there are embedded scrollable pages embedded within pages and the user moves a scrollbar.
screenX and screenY:
Relative to the top left of the physical screen/monitor, this reference point only moves if you increase or decrease the number of monitors or the monitor resolution.
clientX and clientY:
Relative to the upper left edge of the content area (the viewport) of the browser window. This point does not move even if the user moves a scrollbar from within the browser.
For a visual on which browsers support which properties:
http://www.quirksmode.org/dom/w3c_cssom.html#t03
w3schools has an online Javascript interpreter and editor so you can see what each does
http://www.w3schools.com/jsref/tryit.asp?filename=try_dom_event_clientxy
<!DOCTYPE html>_x000D_
<html>_x000D_
<head>_x000D_
<script>_x000D_
function show_coords(event)_x000D_
{_x000D_
var x=event.clientX;_x000D_
var y=event.clientY;_x000D_
alert("X coords: " + x + ", Y coords: " + y);_x000D_
}_x000D_
</script>_x000D_
</head>_x000D_
_x000D_
<body>_x000D_
_x000D_
<p onmousedown="show_coords(event)">Click this paragraph, _x000D_
and an alert box will alert the x and y coordinates _x000D_
of the mouse pointer.</p>_x000D_
_x000D_
</body>_x000D_
</html>