Sitemap is like a list of posts and pages that are present in your website. By creating and submitting the sitemap to Google Webmasters site, you will be able to let know about the pages on your website that is used for crawling.
Different ways of generating Google Sitemap Online
If you are using WordPress, there are some good plugins that generates the google sitemap automatically. What about other websites? Below are collection of some of the ways of generating Google Sitemap online.