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.
At typical Sitemap (usually called Sitemap, with a capital S, also known as Google Sitemap) is an XML file with the following structure:
Of course, the <url></url> section is repeated for each URL you want to include in the Sitemap file.
This example is a static XML file. A dynamic site (based on a database) has to create a dynamic Sitemap (generated and updated by a script, not manually).