Sitemap For SEO
Sitemap is basically all your site URLs listed in any page or XML format. Sitemaps improves the search engine optimization of your site by making sure all you site pages are listed and can be found.
Also you can set what the search engines can crawl and whats not allowing the search engines to crawl efficiently thus helping your site to be indexed in the search engine results effectively.
For more detailed info on sitemaps please refer to Sitemaps.org (set up by Google, Yahoo & MSN)
You can create Google XML sitemaps online here.
I recommend also that you use Google Webmaster tool, been using this from the start of my SEO stint, and by far this helped me a lot.
For Wordpress users, you can also use Google XML Sitemap plugin.
This plugin will create a Google sitemaps compliant XML-Sitemap of your WordPress blog. It supports all of the WordPress generated pages as well as custom ones. Everytime you edit or create a post, your sitemap is updated and all major search engines that support the sitemap protocol, like ASK.com, Google, MSN Search and YAHOO, are notified about the update.
Update: Try also this Sitemap Generator Plugin for Wordpress