A site map is a page (or set of pages) that gives an overview of the layout of a Web site. These originally came about to help people to navigate their way through a Web site or to quickly find a specific page or section of pages found on a Web site. However, in recent years they have become an useful tool for helping with your search rankings.
Search engines send out what are known as ‘spiders’ or ‘bots’. These pieces of code look around the Internet for appropriate sites and then spider them. The term spider means gather information about the pages on the site and send that data to the search engine's data management system.
If the search engine's data management system likes what it sees then your site will appear in their search engine. Conversely, if the search engine data management system doesn't like the data sent back by the spider your site may not appear at all in that search engine.
Where does a Site Map fit into all of this?
Well, a site map is a page that really helps the spider to do its job efficiently, because it provides a link to every relevant page on the site and, if constructed well, it includes the page title and a brief description.
Without a site map the spider would just wander from page to page on your site following the links within the page content itself. Some spiders will dig deep and go down several layers of pages to find relevant content, but many spiders will have a cut off point that is quite shallow such as just one or two clicks deep.
The hope is that the spider will visit your home page, find or stumble across your site map page, and from there find all of the important pages on your Web site with just a single click to each one.
The spider will then visit each of the pages in your site map and add it to the search engine’s huge database of Web pages. This means that rather than have a handful of pages stored in the search engine's database you could end up with hundreds or even thousands, depending on how good your site map looks.