The use of good, updated sitemaps are a good way to tell the search engines what pages are important and should be crawled. Google has recently added some additional information concerning sitemaps. Here’s their announcement from Webmaster Central:
Thursday, January 26, 2012 at 12:18 PM
Sitemaps are a way to tell Google about pages on your site. Webmaster Tools’ Sitemaps feature gives you feedback on your submitted Sitemaps, such as how many Sitemap URLs have been indexed, or whether your Sitemaps have any errors. Recently, we’ve added even more information! Let’s check it out:
The Sitemaps page displays details based on content-type. Now statistics from Web, Videos, Images and News are featured prominently. This lets you see how many items of each type were submitted (if any), and for some content types, we also show how many items have been indexed. With these enhancements, the new Sitemaps page replaces the Video Sitemaps Labs feature, which will be retired.
Another improvement is the ability to test a Sitemap. Unlike an actual submission, testing does not submit your Sitemap to Google as it only checks it for errors. Testing requires a live fetch by Googlebot and usually takes a few seconds to complete. Note that the initial testing is not exhaustive and may not detect all issues; for example, errors that can only be identified once the URLs are downloaded are not be caught by the test.
In addition to on-the-spot testing, we’ve got a new way of displaying errors which better exposes what types of issues a Sitemap contains. Instead of repeating the same kind of error many times for one Sitemap, errors and warnings are now grouped, and a few examples are given. Likewise, for Sitemap index files, we’ve aggregated errors and warnings from the child Sitemaps that the Sitemap index encloses. No longer will you need to click through each child Sitemap one by one.
Finally, we’ve changed the way the “Delete” button works. Now, it removes the Sitemap from Webmaster Tools, both from your account and the accounts of the other owners of the site. Be aware that a Sitemap may still be read or processed by Google even if you delete it from Webmaster Tools. For example if you reference a Sitemap in your robots.txt file search engines may still attempt to process the Sitemap. To truly prevent a Sitemap from being processed, remove the file from your server or block it via robots.txt.