What exactly is an XML sitemap?
In simple terms, it's an incredibly useful file that contains all the URLs on one specific website. An XML, or Extensible Markup Language Sitemap (to give the proper title) often includes metadata about each of the URLs; with practical information; outlining how significant the URL is, when it was last altered and how it relates to all the other URLs within the site.
It's basically a helpful guide for search engines, enabling web robots to trawl your content in a more efficient manner. It also helps notify search engines of any specific changes made on the site; for example, tweaking a page of content, or adding some new text.
Not a guarantee…but helpful none the less
It's important to be aware that an XML sitemap does not guarantee that your site will be crawled and indexed by the search engines. However, overwhelming evidence suggests that it certainly helps increase your chances.
A few useful tags
Here are a few useful tags to use when creating a sitemap for your website.
<loc> : The locator of the page in question.
<urlset>: Protocol standard for opening and closing the sitemap.
<changefreq>: The tag used to determine the frequency of changes within a file.
<url>: Parent tag for every URL
<lastmod>: Information about the most recently modified file.
<priority>: An indicator of the importance, using a value ratio between 0.0 to 1.0.
Some important information about XML Sitemaps
You should always add your XML sitemap to the root directory of your site. Always make sure that all URLs featured on the sitemap come from identical hosts. Also be aware that there is a maximum length for URLs, which is currently set at 2,048 characters.
Since Google announced a preference for fresh, updated content, a few website owners attempted to fool the robots by updating the <changefreq> tag on a regular basis. However, this is inadvisable, and doing so is likely to incur the wrath of Google, who may ignore your sitemap completely as a result.
If you want to use multiple sitemap files for just the one website, this is entirely possible, but you must list all sitemaps in a separate file, which will be named the sitemap index file. For example, if you've got literally thousands of URLs within the one site, or if your sitemap is larger than 10Mb, you may need to create multiple XML sitemaps.
Plenty of file types are supported by XML sitemaps, and will be detected by Google. These include HTML, images, video content, news content and mobile. If you have a lot of images on your site, it makes sense to create an image sitemap, in order to assist the search engines with trawling your site successfully.
Using sitemaps to keep tabs on your site content
There's no guarantee that by having a sitemap you'll be indexed by Google (though it is more likely). However, sitemaps serve a further purpose. Using Google Webmaster Tools (or Bing Webmaster Tools) you can assess which of your pages are indexed against the URLs that you have submitted. If there is a noticeable difference, then you'll know to check it against your sitemap. This is a great way to keep tabs on problems on the site; for example, duplicate content or issues with your robots.txt files.