Sitemaps help either a visitor of a website or a search engine spider find all the pages on a website.
Sitemaps for Human Visitors
These sitemaps are web pages where all the pages on the web site appear as a link, with each link leading to a specific web page. The links are organized depending on the information architecture used on the website with pages falling under each main category.
These pages usually cross link. This cross linking is usually not shown on the sitemap. Its basic function is simply to show the visitor the list of all pages as well as the branching down or hierarchy of the information structure starting from the main page.
Sitemaps for Search Engines
Sitemaps that are used to tell search engines how many and which pages they can crawl on a web site are called XML sitemaps. Instead of being a web page, these sitemaps come in the form of XML files with each file containing a list of pages (URLs) for the site as well as specific meta data on these URLs. These components are added to help search engines known the importance of the contents of the page (relative to the other pages on the site) and how often they need to crawl those pages. XML sitemaps can also be used to tell search engines not to crawl specific pages, which may be important for privacy and/or security issues.