Search engines like Google love content - the more content you add to your site, the more they can include in their search results - a win-win for the search engine and your website. However, search engines have to find the content on your website before they can index it. A site map will make it easier for a search engine to find all of the content that your site has to offer - therefore helping your Search Engine Optimization (SEO) efforts.
Making an HTML sitemap is a simple as creating a page that contains a listing of all of the pages on your website. In order for search engines to find your sitemap, you will need to link to it at least once on your website, preferably on the home page of your website. A sitemap page can also help your visitors find information on your website, especially if has numerous pages or a complex structure that may make it difficult to discover every page.
An XML sitemap is a file that can direct search engines to the content on your website, and is not meant for your website visitors to see. An XML site map file can contain thousands of references to pages on a website, which may not be practical to list individually on a single, publicly viewable HTML webpage. You can also register your XML sitemap with search engines, so they will routinely check it for any new pages you may have recently added.