A sitemap is essentially a map of your website. There are two main types of sitemaps. The first is a html based map, which is designed for your end user to look at to help them find pages on your websites.
The second type of sitemap is an xml sitemap and is designed for submission to the search engines. Technically an xml file is created with extensible markup language and is used for sharing information on the web.
Your xml sitemap gives the search engine information like how important your urls are, when last updated, how frequently it might change and how important the pages are considered to be. This helps the search engines to index your sites.
As we spoke about in our other post about Indexation, webmaster tools also gives you information about your sitemap, which you or your web team would also find useful.