An XML sitemap (typically at /sitemap.xml or split across multiple sitemap files) lists URLs you want search engines to know about, optionally with metadata about last modification date and update frequency. Sitemaps complement (not replace) good internal linking — they help search engines discover URLs they might otherwise miss but do not directly improve rankings. Sitemaps matter most for: (1) New sites with limited inbound links — without external discovery signals, the sitemap is the primary way Googlebot finds your URLs; (2) Large sites (10,000+ URLs) where crawl-budget management is real — sitemaps signal which URLs you consider important; (3) E-commerce sites with frequently changing inventory — sitemaps with accurate lastmod dates help Google prioritize re-crawl; (4) Sites with weak internal linking from the home page — sitemaps surface deep pages; (5) News and publication sites — separate news sitemap signals time-sensitivity. Best practices: (1) Reference your sitemap in robots.txt; (2) Submit it through Google Search Console; (3) Keep it under 50,000 URLs per sitemap file (use a sitemap index for larger sites); (4) Include only canonical, indexable URLs (no redirects, no noindex pages, no parameter variants); (5) Maintain accurate lastmod dates — Google increasingly uses these for crawl prioritization.