What is sitemap.xml?
 

A Sitemap is an XML file that lists the URLs for a site. It allows webmasters to include additional information about each URL: when it was last updated, how often it changes, and how important it is in relation to other URLs in the site. This allows search engines to crawl the site more intelligently.

as per simple language we can say xml sitemap is understandable for search engine... you can easily index your site on search engine..

Re:
 

According to me sitemap.xml is collection of all urls in your site.It is usefull for your site because it helps crawler for crawling.

A site map is a visual or textually organized model of a Web site's content that allows the users to navigate through the site to find the information they are looking for, just as a traditional geographical map helps people find places they are looking for in the real world.

Many people consider XML Sitemaps as an optional extra, but the fact is XML Sitemaps help not only in the indexation of your pages by providing a full list of URLs on your website, but also to provide other useful information to search engines about your site. For example, by submitting “/” as your root domain instead of “default.aspx” through the XML Sitemap, search engines like Google might just figure out “/” is the main URL and this may help with the canonical issues.

Other benefits of XML Sitemaps include:

1- Help improve the crawl rate and indexation on a site
2- Help the crawling of newer content
3- Help spot problems or raise trouble flags (In the case of Google, problems are reported in Webmaster Tools)

In XML Sitemaps, we generally include all the URLs of our site to help search engines discover most of our URLs. We can also create XML Sitemaps for other content like videos and mobile web pages to make them easily discoverable by search engines. In this post, I will cover most of the XML Sitemaps and will talk about videos and mobile sitemaps in a future post.