Sitemaps are similar to a contents section at the front of book. The contents section tells you the name of the chapter and which page it starts on. A sitemap lists the contents of your pages and tells the search engines the correct URL to find those pages.
Site maps come in several formats. The search engines prefer the sitemap created as an XML file. In a recent Q&A session it was also suggested to include a sitemap in HTML format. The HTML sitemap can be used as a page for visitor use as well. If you check around the internet you will find that many sites make use of the sitemap. In fact, you can see a copy of our sitemap and you will get an idea of how they can look, and how much easier it can be for users to find content.
XML sitemaps can be submitted to the search engines. They can then use the sitemap to crawl (visit) your site page by page, following links and indexing pages along the way. If you have just added a few new pages then by submitting your sitemap you may find the search engines indexing those new pages faster than if you had no sitemap.
Blogging software like WordPress and Drupal have plugins or modules that will automatically create new XML sitemaps whenever the content is updated. There is also a plugin for WordPress that will create a HTML sitemap as well.
Take a little time to learn how to create a sitemap for your site and enjoy the benefits of faster and more efficient crawling and indexing of your pages.