HI...
A sitemap is a map of a web site showing the web pages and how web pages are linked/related. Sitemaps are provided so that site visitors can easily navigate your site.
The sitemap doesn't need to include all of your web pages but should include all of the site structure and should fit on a single page. Sitemaps are also used by search engines to index your website. You can create a sitemap by hand coding HTML or by using sitemap tools called sitemap generators.
Here are the Google guidelines for site maps:
"Offer a site map to your users with links that point to the important parts of your site. If the site map is larger than 100 or so links, you may want to break the site map into separate pages".
Google have recently introduced Google Sitemaps which should not be confused with standard sitemaps. The Google sitemaps are intended to be submitted to Google to help the Google spider index your site.