There are two kinds of sitemaps – one for humans and one for the search engines. An html (or php, etc.) sitemap meant for visitors to your site can be an invaluable tool for finding just what they are looking for.
Creating a sitemap – a structured list of all pages of a web site – is especially useful if you are unable to add a search feature to your site. A link to the sitemap is another item that is helpful to place down in the footer of your site, as well. A good sitemap will list out every page of your site in a hierarchial format – clearly showing the relationship of pages in terms of primary pages with sub-pages and sub-sub-pages, etc.
