An HTML sitemap is a dedicated page on a website that contains a visual representation of contents and navigation within the website with a collection of links. In some cases an HTML sitemap can be found in the footer of almost every pages - for more details on the subject, see footer sitemap definition.
Most often HTML sitemaps show text links but more elaborate versions may use links behind graphical elements.
An HTML sitemap or website map has two functions and adresses both humans and search spiders.
HTML sitemap is a usability tools that helps site visitors to easily navigate and find content. This is especially true for small and medium-sized websites, e.g. some B2B websites where navigation and content are not always intuitive. Navigational sitemaps are usually less relevant for large ecommerce websites with several hundreds product categories and millions of web pages.
HTML sitemaps are also precious for SEO purposes. Firstly, they favor search engine indexing by helping spiders to crawl a website. Secondly, they can favorably affect search rankings as they use internal linking and anchor texts which are both good for SEO.
An HTML sitemap should not be confused with an XML sitemap.
A short explanation of HTML interest by Matt Cutts :
An example of HTML sitemap :