XML format ensures that this information can be easily processed on different kinds of computers, applications and systems, so Search engines (Google, Yahoo, Bing, Ask, AOL etc) won't have any problems with understanding your sitemap files.
XML Sitemap sample:
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
<loc>http://www.example.com/</loc>
<lastmod>2005-01-01</lastmod>
<changefreq>monthly</changefreq>
<priority>0.8</priority>
</url>
<url>
<loc>http://www.example.com/about.htm</loc>
<changefreq>daily</changefreq>
</url>
</urlset>