Search engines such as Google discover information about your site by employing software known as “spiders” to crawl the web. Once the spiders find a site, they follow links within the site to gather information about all the pages. The spiders periodically revisit sites to find new or changed content. Google Sitemap Generator plugin helps to generate a Google Sitemaps compliant sitemap of your WordPress blog. It is WordPress 2 compatible. It generates a static XML file in your blog directory and can also create a gzipped version of the sitemap.
An excellent article what Google Sitemaps is and how it can help you to get indexed by Google can be found at the searchenginewatch blog.
If you have questions about the Google Sitemaps program or run into problems, have a look at my Google Sitemaps FAQ.