Following steps are required for showing a popup on mousehover on a link

Step1: Attach the javascript file from http://code.jquery.com/jquery-1.6.3.min.js

Step2: Write the following internal stylesheet under the head section of an aspx page

.tooltip {
width: 25%;
padding: 5px;
background-color: white;
border: 3px solid rgb(195,151,51);
border-radius : 5px;
}
.trigger {
border-bottom: 1px dashed white;
cursor : help;
}
.trigger:hover {
color: rgb(255,0,0);
}

Step3: Write following script under head tag of an aspx page

script type="text/javascript">
    $(document).ready(function () {
        $('.trigger').mouseover(function () {
            var ttLeft,
 ttRight,
 $this = $(this),
 $tip = $($this.attr('data-tooltip')),
 triggerPos = $this.offset(),
 triggerH = $this.outerHeight(),
 triggerW = $this.outerWidth(),
 tipW = $tip.outerWidth(),
 tipH = $tip.outerHeight(),
 screenW = $(window).width(),
 scrollTop = $(document).scrollTop();
            if (triggerPos.top - tipH - scrollTop > 0) {
                ttTop = triggerPos.top - tipH - 10;
            } else {
                ttTop = triggerPos.top + triggerH + 10;
            }
            var overFlowRight = (triggerPos.left + tipW) - screenW;
            if (overFlowRight > 0) {
                ttLeft = triggerPos.left - overFlowRight - 10;
            } else {
                ttLeft = triggerPos.left;
            }
            $tip.css({
                left: ttLeft,
                top: ttTop,
                position: 'absolute'
            }).fadeIn(200);
        }); // end mouseover
        $('.trigger').mouseout(function () {
            $('.tooltip').fadeOut(200);
        }); // end mouseout
    });                //end ready

Step 4: To test the script is working or not create a link under the body tag