A Sitemap is usually used for the purpose of letting the search engine crawlers follow the links to all your individual webpages so that it won't miss out on anything.
I guess you know better about Sitemap. :)
You can try check this article about all kinds of sitemap file types:
http://www.microsystools.com/products/sitemap-generator/help/website-sitemap-kinds-comparison/ (http://www.microsystools.com/products/sitemap-generator/help/website-sitemap-kinds-comparison/)