There are 2 common types of sitemaps. The first is a HTML page listing the pages of your site. It helps users locate information. The second Sitemap is in XML and this  provides Google information about your site.

A Sitemap lists pages on your website. A Sitemap ensures that Google knows about all the pages on your site. This includes  URLs not found its normal crawling.

Sitemaps are specially useful if your site:

• has dynamic content.

• has pages not easily discovered by Googlebot during the crawl  - for example, pages with Flash.

• has a large archive of content pages which are badly or not that are not well linked

• is new and has few links to it. (Googlebot follows links, so if your site isn't well linked, it may be hard to discover it.)

You can also use a Sitemap to provide Google with more information, including:

• How often pages change. For example, the Product page is changed daily, but  Contact Us almost never.

• Last date a page was last modified.

• The relative importance of pages on your site. Your home page might have a relative importance of 1, category pages 0.8, and individual blog entries 0.5. This priority indicates the relative importance of URLs. It doesn't impact the ranking of pages in search results.

Sites are never penalized for submitting Sitemaps. They help Google crawl more of a  more often, but it is not certain if Google will add URLs from the site to the Google index.  

Google subscribes to Sitemap Protocol 0.9 in sitemaps.org. The Protocol is a dialect of XML to summarize Sitemap information relevant to crawlers. Sitemaps using Sitemap Protocol 0.9 are  compatible with other search engines that adopt the sitemaps.org.

A standard Sitemap works for most sites. Specialized Sitemaps are specific to Google and are not used by other search engines. Sitemap formats include:

• Video Sitemaps

• Mobile Sitemaps

• News Sitemaps

• Code Search Sitemaps

• Geo Sitemaps