It's called a tool tip.
refer to http://www.publishermvps.com/WebDesign/Advanced/tabid/97/Default.aspx

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David Bartosik [MSFT MVP]
www.publishermvps.com
> I want to have text appear (like a tooltip) when you place the mouse cursor
> over a hyperlink. I've added ALT text, but that just shows up before the
> graphics load. Is there any way to add these "pop-up" descriptions?