Your Web site is simply a means of providing access to information. As such, the single most important aspect of any site is its navigation. Bad navigation leaves the user confused, stranded, and frustrated.
For an overview of factors that make your site user-friendly, see Improve the Customer Experience on Your Web Site.
The first step in designing a navigation system that works is deciding where on the page to place it. For a long time, it was an unwritten rule of site design that navigation lived in the left-hand column. Then, with the emergence of more sophisticated design tools such as Cascading Style Sheets, some sites used the header for navigation. Doing so freed up space in the body of the home page for content and led to a new wave in site design.