Drop down menus on web pages add an elegant style to the presentation, but they can be very quirky. Commonly, the menus are build with lists and using CSS that control the visiblity of list items based styles estalished for the the hover property in CSS.
Another method that I have seem demonstrated quite eligently at The Ojibway Club is to use a combination of tables and divs. That site was developed by The Big Brain.. The source code for that page shows a very clean layout of the menus. This was created with Joomla in conjunction with a third party joomla menu system.