What's the difference between an XML and HTML sitemap?

An XML sitemap is a machine-readable file consumed by search-engine crawlers. An HTML sitemap is a human-readable webpage. The XML version uses the sitemaps.org protocol — XML tags, lastmod dates, optional priority — and lives at /sitemap.xml. The HTML version is a normal page (often at /sitemap or /sitemap.html) listing every internal link as anchors, designed for visitors. XML sitemaps are the SEO essential; HTML sitemaps are an accessibility and UX nicety. They don't replace each other. When converting to CSV, the two formats need different parsers — xmlsitemapmaker.com auto-detects which one you've given it, parses XML via the urlset structure or HTML via anchor extraction, and produces the same CSV format either way.