In general, there are two types of sitemaps.
The first type of sitemap is a HTML page listing the pages of your site - often by section - and is meant to help users find the information they need.
While this type of sitemap can help both visitors and search engines navigate your site, it's different from an XML Sitemap that can be submitted to Google using Webmaster Tools.
XML Sitemaps - usually called Sitemaps, with a capital S - are a way for you to give Google information about your site.
Sitemaps are in my opinion a mandatory item if for nothing else a search engine. Having all your pages indexed is a priority and a site map tells the search engines where to find the pages.
Do a google search for "google sitemap" and you'll be able to get one generated for free on your site. Upload it to your google webmaster section for your site and you'll see instant results.
Other search engines such as Yahoo and Bing also have sitemap upload areas that are effective.
Using Site maps are important on websites where:
1)some areas of the website are not available through the brows able interface, or
2)webmasters use rich Ajax, Silverlight, or Flash content that is not normally processed by search engines.
The webmaster can generate a Sitemap containing all accessible URLs on the site and submit it to search engine.