Can anybody tell me how sitemap is useful in crawling and indexing website in search engine and also tell me the difference between XML sitemap and HTML sitemap?
It can help speedup discovery of new/all site URLs. See also http://www.microsystools.com/products/sitemap-generator/help/website-sitemap-kinds-comparison/ (http://www.microsystools.com/products/sitemap-generator/help/website-sitemap-kinds-comparison/) for explanations on differences between HTML, XHTML, XML, RSS etc. sitemaps