<!-- This sets up a button with a help icon next to it -->
<input type="button" id="cascade" class="StandardButton" value="Cascade" />&nbsp;<img id="help" src="~/Images/help_icon.jpg" border="0" />

<!-- This is the jquery code to add a tooltip to the help icon -->
$(document).ready(function() {
 $('#help').tooltip(
 {
    delay: 0,
    bodyHandler: function() { return "Click this button to cascade your selection<br /> from its current level through all levels below."; },
    showURL: false
 })
});

I know that there are other ways of displaying a tooltip without the use of JQuery but I like the flexibility and control JQuery provides. This snippet shows how to add a tooltip to a Help icon.

JQuery: Displaying a Tooltip

jQuery