Sitemap is basically the map that contain information of all located resource on a website. It helps users, as well as search engines, to easily explore a website for their required object. On the other hand it also helps them to improve their search (for user) and indexing experience (for search engines).

There are principally two types of sitemaps;
HTML sitemap (written in Hypertext Markup Language)
XML sitemap (written in Extensible Markup Language)

XML Sitemaps can have two types.
Index Sitemap (how many URL sitemaps a website have)
URL Sitemap (contain final information of URLs on webpage)