What are sitemaps?
Sitemaps are a standard way of informing search engines about your site content. Sitemaps facilitates and speeds up search engine crawling of your site. This means that new content on your site will appear pretty quick on the search engine results.

Sitemap is nothing but a simple XML file which lists URLs on your site. It also includes URL meta data such as the date when it was changed, frequency of change and how important the content is for your site.

Till last year, there were different specifications for sitemaps. But now there is only one specification supported by Yahoo, Google and Microsoft. At last they agreed on something! Following a sample sitemap XML.

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
   <url>
      <loc>http://www.example.com/</loc>
      <lastmod>2005-01-01</lastmod>
      <changefreq>monthly</changefreq>
      <priority>0.8</priority>
   </url>
</urlset> 

How do I create a sitemap for my site?
If you are hosting account with a webhost then you can use Google’s sitemap generator. Please note that this requires Python support on your webhost and you should be able to run Python scripts on it. Google also gives a list of other sitemap generators.

If you are using a blogging tool I suggest you use the plugins that are available. This ensures that as soon as you post a new entry, sitemap is updated and Google is notified about the change. On my Wordpress installations I use Arne Brachhold’s Google sitemap generator for Wordpress which offers hassle free generation of sitemap.

There are a couple of online sitemap generators which can generate a sitemap for an existing website. But most of these free versions have a limit of 500 URLs.

Sitemap restrictions
A single sitemap should not be more than 10MB and is limited by a maximum of of 50,000 URLs. If your sitemap file is more than 10MB, you should break up it into multiple sitemaps and then provide a sitemap index file which lists the sitemap files.

Sitemap tips
Every time a search engine is informed of a change in sitemap in your site, it will download the entire XML. Hence for large sites you should compress the sitemap to a zip file.

You can submit your sitemaps to Google and Yahoo. Both these services require you to sign up. I prefer Google webmaster services since it offers additional features such as page ranking of your URLs. Following is how this site appears on google sitemaps,

References
Sitemaps official site
Google sitemap generator
Arne Brachhold’s Google sitemap generator for Wordpress