If you want the description to be displayed on the mouseover , and not on the change , I think it's better to use tooltips. This can be done using the HTML attribute title, no JS required.
Example:
<input name="selectedValue" id="selectedValue">
<select id="TestCombo" name="TestCombo" >
<option title="Nothing." value="0" selected="selected">Zero</option>
<option title="The smallest number that has a meaning." value="1">One</option>
<option title="Look, another small number!" value="2">Two</option>
<option title="RGB - Red-Green-Blue. That three colors!" value="3">Three</option>
</select>
source
share