Google have started a new (June 2005) project called Google Sitemaps. The idea behind it is to give the Googlebot a helping hand when indexing your website. We are told that by adding a sitemap we allow for more efficient and tailored searches. Allowing the search engine to harvest links that it may otherwise miss.
According to Google using a sitemap will not give higher search rankings and still does not guarantee search placement.
So now for the technical bit. A site map is basically an XML document made up of links (URL's) from your site, obviously the more links the more accurately it will map to your website.