There’s quite a bit of confusion around the web as to what a site maps are and how important they are to your website’s SEO and/or functionality. Here it is, short and sweet:
HTML Site Map (site map is two words): Is a page within your site that reader’s can go to to quickly find things. It is for the benefit of people – not search engines. In fact, I read that Google doesn’t particularly like a page that has more than 100 links. But I don’t think its much of a penalty.
Most people seem to think that these have to be custom made. Certainly not! I have better things to do with a full day than create site maps! I use the WP Sitemap Page plugin to generate mine. The downside is that there isn’t much control over what is and isn’t generated, but that isn’t as important to me as the efficiency of the plugin.
XML Sitemap (one word): This is a file that is generated and kept within your directory for the benefit of search engine robots. Its a listing of the pages in your website that help search engines find pages that may otherwise be difficult to find, such as pages that aren’t linked well. It helps the robots learn about the structure of your site.
It really isn’t a huge benefit as sooner or later your pages will more than likely get indexed. But I did find it useful on one of my sites when I accessed my Google Web Tools and found that many of my pages weren’t getting indexed due to some duplicate metadata.
I generally use the XML Sitemap feature that comes along with Yoast’s SEO plugin, but you can also simply use Google XML Sitemaps if you’re using a theme with good built in SEO features.
To sum it up: Both site maps are somewhat useful, so use them if you have a little time and a little technical knowledge.