A sitemap is a diagrammatic representation of a hierarchical system. It usually depicts the parent-sibling relationship between pages in a website, showing how sub pages might be arranged underneath their parent groupings. This arrangement forms a map of the site.
Sitemaps as a design project deliverable are documents that demonstrate the navigation layout and organisation of content within. A sitemap as a functional XML file on a website enables search engine crawlers to build a list of page to index (or not). A sitemap can also be an HTML page on a website listing every page within that website for a user to navigate.
In a website design project, a sitemap is used to plan the organisation of pages to improve usability in the early planning stages, often before any wireframes or UI Designs are created. Exercises involving card sorting can be used to help determine the ideal sitemap structure. Navigation prototypes can be also be created to test assumptions about the logical layout with users to validate any assumptions.
An example of a sitemap from a recent website project: