What is Google site map?
Sitemap is the best way to inform Google crawler about the web pages and data within you website. This is very effective way to get indexed the web pages of website. Sitemap is created using xml and html but remember that xml should need to submit in webmaster tool and html should need to be for visitors.
I think you are asking about xml sitemap. It is used to help the search engine to crawl well.
Google sitemap is map of your site which contain all the pages and posts of your site, categories as well.