Basically this is a combination of CSS and Javascript . The CSS positions the 'div' block elements in page and makes them invisible , and javascript dynamically makes them visible using a onMouseOver event .

But this is really an effect which can give you headaches for all eternity , due to different DOM implementations in browsers . Just try accesing the UT menus with three different browsers , like I do . Result : 3 different behaviours . If you're still interested , i can give you code for a simple menu .