What is a Sitemap file and why should I have one?

A Sitemap file lets you tell google about all the pages on your site, and optionally, information about those pages, such as which are most important and how often they change. By submitting a Sitemap file, you can take control of the first part of the crawling/indexing processes: our discovery of the pages.
Sitemap is an XML file that lists URLs for a site along with additional metadata about each URL so that search engines can more intelligently crawl the site.