An XML Sitemap is essentially a text based URL road map of your website that search engines can use to more easily index your website and the pages within it more intelligently and accurately. Adding a Sitemap does not necessarily mean that your site will be indexed but it is recommended by Google (which is reason enough for me to include it). XML Sitemaps are a URL inclusion tool, unlike robot.txt files which are URL exclusion or exception tools. Together they help you organize your site so that it is optimized for search engine spiders who crawl your site.
XML Sitemaps for HTML/CSS based sites
In order to add an XML Sitemap to a traditional html/css based website (not built on a CMS) you will need access to the domain root folder of your site. This may be best left to your web design/development company, web marketing consultant, or SEO provider. If you feel comfortable creating a XML Sitemap for your site, and have ftp access to your site's root folder, there are several tools( a Google search returned 23,300,000 results for [free xml sitemap]) for creating an XML Sitemap. For ease of use, I generally use http://www.xml-sitemaps.com. If you are really feeling froggy and want to write your own XML Sitemap, there are resources for this as well, including http://sitemaps.org/protocol.php.
It should be noted that when setting up your XML Sitemap, you type in the web address as it is being optimized. For example, http://your_site_name.com may not be the same as http://www.your_site_name.com. Check with your SEO consultant or web developer to see which URL is being optimized for (the other should 301 redirect) and be sure to create your Sitemap around that web address.
XML Sitemaps for Wordpress
If your site is one of the many websites now being built on the Wordpress format, installing an XML Sitemap can happen right from the admin panel. The Google XML Sitemap plugin can be activated in a couple of clicks. Adding plugins use to be a pain, but with the improvements to the Wordpress CMS, you can now search for the plug-in under the 'Plugins' tab in your admin panel, and then install the plugin without ever leaving your admin panel. (Just make sure to read and follow the on screen instructions).
Other CMS systems have similar functionality, allowing you to add an XML Sitemap fairly easily.
For the more advanced users, there are certain criteria (like modification date, update frequency, priority, etc) which can be added in the XML file. These criteria are optional. For an explanation of the XML tag definitions you can go here.
In summary, an XML Sitemap is an great way to help search engine spiders organize your URLs for inclusion in their search engines. All of the major search engines (Google, Bing, Yahoo!) support XML Sitemaps.
Too nerdy? I try to provide important information for small to mid-sized business owners to help them improve their online marketing initiatives. Let me know if this post was helpful for you.