Created typically in a hierarchical fashion, a site map is submitted to Google for better Crawling. An XML Sitemap is directory you create that is a list of pages of your website assembled in a list. This can be done in any document or web planning tool.
A sitemap is an XML document on your server that lists each page on your website. It tells the search engines about pages which are added and how often to check back for new additions to your site. This encourages Google to come back and crawl your site on a regular basis and may increase your results due to the increased activity it notices on your page.
Google may have a difficult time crawling your website if they are unable to clearly identify the pages and URL’s of the individual pages within your site.
When your site has pages that are dynamic, pages with rich AJAX or images, or if your site is newer and has few backlinks, or a site that has a lot of archived pages with little or no linking, a Sitemap can better direct Google.