Definition:
Yahoo! Site Explorer is a web tool developed by Yahoo! to help streamline the submission of webpages to its search engine database. Submitting a sitemap allows search engines to have a complete list of every webpage associated with your website, and that is very important for ensuring site visibility.
Google offers a similar tool called Google Sitemaps to allow webmasters to submit a complete listing of their web pages to the Google search engine. Both submissions utilize the sitemaps.org XML protocol standard.