An XML sitemap is essentially a roadmap of your website written in XML format that you submit to search engines through Google Search Console, Bing Webmaster Tools, or other platforms. It lists URLs you want indexed along with optional metadata like last modification date, change frequency, and priority level, though Google mostly ignores those metadata fields now. The primary function is discovery, not ranking. If your site has strong internal linking, Google will find most pages without a sitemap. But sitemaps become critical when you have orphaned pages, deep page hierarchies (4+ clicks from homepage), large sites with thousands of URLs, new sites with few backlinks, or pages that update frequently like news articles or product listings. For the 500+ domain portfolio we manage at Ottawa SEO Inc., we generate sitemaps dynamically through CMS plugins or custom scripts. A typical sitemap can contain up to 50,000 URLs and be 50MB uncompressed, but we usually split large sites into multiple sitemaps using a sitemap index file. E-commerce sites might have separate sitemaps for products, categories, and blog posts. Common mistakes include listing noindexed pages, redirect chains, pages blocked by robots.txt, or duplicate URLs with different parameters. Your sitemap should only contain canonical, indexable URLs that return 200 status codes. We also exclude thin content pages, pagination, and filtered views that don't deserve indexing priority. Submitting a sitemap doesn't guarantee indexing. Google uses it as a hint but still decides independently what to crawl and index based on crawl budget, content quality, and site authority. Check your Search Console coverage report regularly because Google will flag errors like submitted URLs marked noindex or returning 404s. For most business sites under 500 pages with clean architecture, a sitemap is insurance rather than necessity, but it takes 10 minutes to set up properly so there's no reason to skip it.