There are two different kinds of sitemaps:
1. The visual ones. These contain all the links to all your pages that you want indexed by SE. Usually a link to the
sitemap page is contained in the homepage.Once a SE has found your dite, it also discovers (from the homepage link) the sitemap page and from there all your pages URLs. This is a human sitemap, but serves as well for SEs too.
2. The SE sitemaps. These are only for SE use, and yes, they are usually in xml (like ROR). It does the same job, but only for SEs.