goodroi - 12:36 pm on Jun 14, 2010 (gmt 0)
many, many years ago webmasters were having a hard time getting all the content indexed. they started to create a sitemap.html page. this page was simply a list of links to all the pages on the site. this helped search engines & users. however it was limited since it was html.
the search engines got together and agreed on a formal sitemap protocol. this one focused on xml files. this made it easier to upload sitemaps of thousands of urls and include bonus details like last modified. for more details about this check out the official site [sitemaps.org....]
you can have both html & xml. the html should be more for users and the xml should be for the search engines.
i wouldnt get too worried about either sitemap variety since if you have enough links you will not even need a sitemap. even if a sitemap gets more of your pages indexed, they won't rank for any competitive term without backlinks. sitemaps are helpful but imho you should keep your efforts in perspective and not forget the value of links.