With modern web standards, it shouldn't be that hard: just use the :hover CSS pseudoclass. Unfortunately, to make a long story short, Internet Explorer 6.0— still a very popular browser - does not support :hover, except on links. And you can't nest links... and menus without links aren't very useful!
So I'm not going to duplicate their excellent work here. The best and cleanest implementation of dropdown menus for all browsers is found in the article Son of Suckerfish Dropdowns, by Patrick Griffiths and Dan Webb. So check that link out now and you'll be up and running with dropdown menus (and even nested dropdown menus!) in no time. You should benefit from their hard work and not try to reinvent the wheel.
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