In its simplest terms, a Sitemap is a list of the URLs on your site. Creating and submitting a Sitemap helps make sure that Google knows about all the pages on your site, including URLs that may not be discoverable by Google's normal crawling process. You can also create a single Sitemap that lists all URLs you want Google to immediately index for your custom search engine.
Google can accept Sitemaps in a number of formats, but we recommend creating a Sitemap based on the Sitemap protocol because the same file can be submitted to the other search engines, such as Bing and Yahoo!, that are members of sitemaps.org. More information about creating a Sitemap.
We recommend using the following tags in your Sitemap, as they can help Custom Search identify the most important pages for indexing:
<priority>: This tag tells search engines about the relative importance of pages on your site.
<lastmod>: This tag tells Googlebot when the content was last updated.
Once you’ve created a Sitemap, upload it to your server, and then submit it to Google using Webmaster Tools. (You can submit your Sitemap using ping and robots.txt, but submitting it using Webmaster Tools gives you access to Google diagnostic data, which can be very useful in troubleshooting problems.)