creating tool tip on mouse hover

In a form I have a drop down box and a multiple select box.Depending on the value of drop down box,the multiple select box is populated using java script.i.e. when you change the drop down box,the multiple select box also changes its contents.

It's working fine now...
I would like to add something more to it...when the user brings his mouse over the value in the multiple select box,a description of the item must be seen either as a tooltip or as a message in a particular portion of the page...

Help me please...