Difference between XML & HTML site map ?

A XML sitemap protocol is specifically intended for search engine spiders. At its root, XML is a file that includes all the behind the scenes activity on a web site. Not just the site’s main URL, but all the URLs within the site along with the associated metadata. This can include when the URL was last updated, how important it is, the average frequency changes occur, the URLs relation to the rest of the site, etc.

HTML is just a general overview of the site, just the pages and info a user needs to be concerned with. If you’re on a web site and you’re looking for the shopping cart or the ‘Contact Us’ page and can’t find it, you’d go to the sitemap and easily find it there. While this is geared towards the user, it can also help your search engine ranking because your site is user-friendly and catering to the site visitor