What is search engine sitemap and why it is important?

Accordingly to Google Webmaster Guidelines;

"Sitemaps are a way to tell Google about pages on your site we might not otherwise discover. In its simplest terms, a XML Sitemap–usually called Sitemap, with a capital S–is a list of the pages on your website. Creating and submitting a Sitemap helps make sure that Google knows about all the pages on your site, including URLs that may not be discoverable by Google's normal crawling process."

Accordingly to sitemaps.org;

"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 (when it was last updated, how often it usually changes, and how important it is, relative to other URLs in the site) so that search engines can more intelligently crawl the site."

"Web crawlers usually discover pages from links within the site and from other sites. Sitemaps supplement this data to allow crawlers that support Sitemaps to pick up all URLs in the Sitemap and learn about those URLs using the associated metadata. Using the Sitemap protocol does not guarantee that web pages are included in search engines, but provides hints for web crawlers to do a better job of crawling your site."

Reference:

http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=156184
http://www.sitemaps.org/

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