All but the most limited of websites must have a mechanism by which visitors can navigate around the pages of the site from the home page. In order to meet accessibility guidelines, several methods of navigation will usually be available, including at least a navigation menu and a site map.
There have been many different implementation styles that have been popular over the years. Before anyone really worried about accessibility or standards compliance, a common way of designing a navigation menu was to use a series of images that linked to other pages of the site, and there was also the popular frame-based navigation structure. While these methods saved the designer a lot of time, effort, and any real skill, they led to hugely increased page load times and a legacy of bad coding practice.
Thankfully, those days have long since passed, and with the continued development of CSS, it's now possible to design an effective navigation structure based on semantic HTML...