A site map is complete list of all pages within a whole website.
These site maps are often formatted as an XML file, structured in a way to help web crawlers easily find all pages to look through.
XML site maps can be styled with CSS, in order to be more both more visually pleasing, but also much more readable to humans.
Some websites also include a dedicated page for a human-friendly overview of the entire site.
Site maps are critical to SEO.