An Xml site map is simple xml file which contain information about urls of your website. In xml site map we can define the priority of every page. From this search engine can easily read your web pages.To get xml sitemap generated simply type in URL and on the next page make selections for 3 optional parameters - page changing frequency, last modified date and page priority.
For me sitemaps are very important in seo. crawler before crawl any web site first thing it try to search for robot.txt and sitemap of particular website. robot.txt to know prohibited information in the web page and sitemap to know what are the information web site consist and its path. which make website more search friendly.
Sitemap is a selection of your whole websites. If your create sitemap in XML structure and publish to Google web master resources then search engine optimization spider comes your site's whole webpages and catalog it easily.
Usually site map has two types, XML and HTML, difference between XML and HTML. XML is good for search engines and and search engine easily capture your all data through XML and HTML is good for Visitors view.
XML site map is the best.....