Quick answer - A sitemap is a map of your site. It helps in better indexing of the website. Make a search about it on Google ( or on DP ) and you will find some nice information.
sitemap is the webpage where search engine can found all the website internal pages link at one location, and crawl the site all pages easily.
Bloomtools, I think there is a speech about XML sitemaps not html sitemaps. salmanq, may be sitemaps.org will help you or en.wikipedia.org/wiki/Sitemap#XML_sitemaps
There are many forms exist in sitemaps like siemap.xml, siemap.html and ror.xml. The first one is meant to remind the google about letting the bot visit your site since you have updated your content. The second one serves as a guide to the visitors about the structure of your site. The third one ror.xml is similar to RSS feed. You can generate all the three sitemaps free of cost by using the tool http://www.xml-sitemaps.com