"Sitemaps are an easy way for webmasters to inform search engines about pages on their sites that are available for crawling. In its simplest form, a Sitemap is an XML file that lists URLs for a site along with additional metadata about each URL (when it was last updated, how often it usually changes, and how important it is, relative to other URLs in the site) so that search engines can more intelligently crawl the site." - sitemap.org
The downside of using the XML Sitemap is that we are not listing anchor text to be used as keyword associations. So for modest sized websites we might prefer to use a simple html file that lists all your links with appropriate keyword anchor text. Another advantage to the html sitemap is that it levels a site to 2 or 3 levels.