HTML Sitemap
A HTML sitemap is simply a hierarchical list of links on a web page that helps web visitors review in a quick glance a site’s index of pages, which helps promote site usability.  A well built HTML sitemap will highlight pages of a site with great importance and diminish 2nd or 3rd tier pages, making it easy to search an entire website from one web page.  See an example of a HTML sitemap.

Resource: HTML Sitemaps Guide

Image Credit: eVisibility

XML Sitemap
XML sitemaps are created for search engines to inform them about URLs that are available on a website to be crawled.  For large sites or sites that have accessibility issues, can make it difficult for search engines to index all of its pages.  By submitting a XML sitemap, it will help search engines find all of those pages that can be nested a few directories deep.  See an example of a XML sitemap.

Resource: SEOmoz – XML Sitemaps: Guideline on their Use

ROR Sitemap
A XML variant, ROR sitemaps are used to describe any object on a website.  They are much more robust than XML because it supports multiple formats including URLs, products, articles, company bios, events, etc.

To create a ROR sitemap try ROR Sitemap Generator or Cumbrowski’s Free ROR Sitemap Generator.

Video Sitemap
A Video sitemap contains links to a videos landing page, along with information that is essential for indexing the video to make it searchable for sites like Google Video. To submit a video sitemap to Google there are certain fields that are required including the video url, the video and video player location, a video thumbnail, and a video title and description.  Google can crawl the following formats; .mpg, .mpeg, .mp4, .mov, .wmv, .asf, .avi, .ra, .ram, .rm, .flv.

Geo Sitemap
Google recognizes Geo Sitemaps that enable you to publish geo-content that can be searchable in Google Maps and Google Earth.  Geo-content can be in KML or GeoRSS format.  Be sure to add the attribution tag which appears in the Google search results for your content.

News Sitemap
News sitemaps allow webmasters to have more control over their content that gets submitted to Google News.   You can also add additional information about your news, like article title, publication date, and keywords.  Using a news sitemap is perfect for websites that are new, have dynamic content, or large sites that can be hard to find content.  Keep in mind that Google only allows you to submit content from the last three days.

Mobile Sitemap
Mobile sitemaps allow you to submit a list of URLs with content specifically designed for mobile devices and is used in the search engine’s mobile index.  Searching the mobile web is different than the regular web, for instance it only displays sites that are “mobile friendly.”  Using well-formed markup like WML, cHTML, XHTML Basic or XHTML MP formats will ensure your content gets crawled.  See an example of a Mobile Sitemap.

Recommended Sitemap Resources

Google Sitemap Generator
Allows you to create XML, News, Mobile, Geo, HTML, Text, ROR, and Video sitemaps. Unlike 3rd party sitemap generators, the Google Sitemap Generator will monitor your website content and update automatically.

AuditMyPC – XML Sitemap Generator
Create unlimited XML and HTML sitemaps.  You must have Sun’s Java Runtime installed in order to create a sitemap.

XML Sitemaps.com
Allows you to create XML, ROR, Text and HTML sitemaps, up to 500 pages (Free Version) or Unlimited pages for the paid version.  Paid version cost $19.99.

Google XML  Sitemaps for WordPress
This wordpress plug-in is by far the best sitemap generator.  Nathan Rice has a great post on the Ultimate Guide to WordPress SEO that goes into great detail of the plugin and the process.