A 'menu' is a website navigation paradigm drawn from normal desktop applications. One moves their mouse over an element, does something, and more choices drop down so that the user can have a broad range of choices of action. Menus collect related choices together so that you have a branching effect from general choices to specific choices.
It's useful to consider the capabilities of the various web browsers. Not every browser properly renders the techniques given in these pages. As browsers improve compatibility, the techniques will work in all generally available browsers. However so long as the old browsers are still in use, this means web developers have to test on various browsers. Some of these techniques degrade properly in older browsers such that the user doesn't see a dropdown menu but still sees the whole of the navigation structure.
It's useful to make sure the site remains accessible. Not everybody can see or hear as well as you do, and there are legal requirements in many places that websites must be accessible. Accessibility means that the site offers extra information snippets so that people with disabilities can use special equipment or software to aid them to navigating the web site.
Killer CSS Navigation: Everything You Need to Design Sweet CSS Menus: A compendium of CSS based navigation tutorials and methods.
CSS Horizontal Drop Down and Pop Out Menu using Peterned's csshover.htc is a tutorial on creating a "horizontal" menu using pure CSS. Horizontal means the labels of the menu are in a horizontal line, and the menu panes drop down below. The same site has other tutorials such as a vertical CSS menu.
DD Tab Menu (5 styles) is several forms of horizontal menus developed using CSS and definition list elements. On Menu and Navigation they have several other tutorials and the site is chock full of other ideas. Chrome CSS Drop Down Menu (v2.01)
CSS menus is a detailed tutorial taking you step-by-step through designing a CSS menu.
Hoverbox Menu is a detailed tutorial taking you step-by-step through designing a CSS menu (of sorts). The results they show are hideous, but the code they use is key. It's not quite a menu, however, because there are not dropdowns.
CSS Menu Maker is a set of predesigned pure CSS menu modules you can download and customize for your needs.
Dropdown low down is a listing of several CSS menu implementations, as well as a discussion of pro's and con's of each.
Advanced CSS Menu is a methodology for a very fancy looking design oriented for constructing a CSS menu.