XML Sitemaps - dynamically created
Sitemaps are an easy way for webmasters to inform search engines about pages on their sites that are available for crawling. In its simplest form, a Sitemap is an XML file that lists URLs for a site along with additional metadata about each URL... http://www.sitemaps.org
Benefits to using a xml sitemap
The first set of benefits revolve around being able to pass extra information to the search engines.
- Your sitemap can list all URLs from your site. This could include pages that aren't otherwise discoverable by the search engines.
- Giving the search engines priority information. There is an optional tag in the sitemap for the priority of the page. This is an indication of how important a given page is relevant to all the others on your site. This allows the search engines to order the crawling of their website based on priority information.
- Passing temporal information. Two other optional tags (lastmod and changefreq) pass more information to the search engines that should help them crawl your site in a more optimal way. "lastmod" tells them when a page last changed, and changefreq indicates how often the page is likely to change.
Being able to pass extra information to the search engines *should* result in them crawling your site in a more optimal way. Google itself points out the information you pass is considered as hints, though it would appear to benefit both webmasters and the search engines if they were to use this data to crawl the pages of your site according to the pages you think have a high priority. There is a further benefit, which is that you get information back.
- Google Webmaster Central gives some useful information when you have a sitemap.
Source: SEOmoz
