A sitemap is a listing or a presentation of most of the pages in a website, in some sort of structure. It's sort of like the index in the back of a book. People often look for a sitemap if they can't easily find what they want in the website.

Google's XML Sitemaps are an advanced technique in Search Engine Optimization.

XML is an acronym that sounds like a satellite radio, but it's just eXtensible Markup Language. An XML Sitemap is a sitemap written for computers and search engines to read, and not for people to read. An XML Sitemap is a specially coded document that tells Google what pages you have, when they've been updated, and how important they are to your overall website.

The XML Sitemap takes some work, and it makes things easy for Google and other search engines (Yahoo, Microsoft) that support the sitemap.org protocol. In return for your effort, if you update your site and the changes are reflected in a new XML sitemap, Google will probably re-index your new content in a day or two, rather than a month or two. It's a significant benefit, especially for complex or competitive web sites.