What is a Sitemap?
A site map (or sitemap) is a list of pages of a web site accessible to crawlers or users. It can be either a document in any form used as a planning tool for web design, or a web page that lists the pages on a web site, typically organized in hierarchical fashion. This helps visitors and search engine bots find pages on the site.
Why use a sitemap:
Using sitemaps has many benefits, not only easier navigation and better visibility by search engines. Sitemaps offer the opportunity to inform search engines immediately about any changes on your site. Of course, you cannot expect that search engines will rush right away to index your changed pages but certainly the changes will be indexed faster, compared to when you don't have a sitemap.
Also, when you have a sitemap and submit it to the search engines, you rely less on external links that will bring search engines to your site. Sitemaps can even help with messy internal links - for instance if you by accident have broken internal links or orphaned pages that cannot be reached in other way (though there is no doubt that it is much better to fix your errors than rely on a sitemap).
If your site is new, or if you have a significant number of new (or recently updated pages), then using a sitemap can be vital to your success. Although you can still go without a sitemap, it is likely that soon sitemaps will become the standard way of submitting a site to search engines. Though it is certain that spiders will continue to index the Web and sitemaps will not make the standard crawling procedures obsolete, it is logical to say that the importance of sitemaps will continue to increase.
Sitemaps also help in classifying your site content, though search engines are by no means obliged to classify a page as belonging to a particular category or as matching a particular keyword only because you have told them so.
Having in mind that the sitemap programs of major search engines (and especially Google) are still in beta, using a sitemap might not generate huge advantages right away but as search engines improve their sitemap indexing algorithms, it is expected that more and more sites will be indexed fast via sitemaps.