What is a sitemap?

Mostly used sitemap is in HTML format. It contains list of all links present on your website. This sitemap is generally for visitors of the website to find the desired links quickly and it also helps search engine spider to crawl and indexing of pages on your website.
Other type of sitemaps found in ROR, text, XML formats.

What is XML or search engine (Google, yahoo) sitemap?

Recently biggest search engines Google & yahoo introduces own standard of sitemap i.e. XML sitemap and now ask.com also support this format. Sitemap file include the lists of all the URL's along with changes made for each, change frequency & priority. Synonyms for sitemap are crawler-pages, gateway-pages, hallway-pages, spider-pages.

Picture showing XML Sitemap

This is How XML Sitemap Looks

Why XML sitemap?

Apart from the URLs sitemap contains other metadata about links like change frequency, last modification date, priority is also the part of sitemap.

Second thing, Visibility and indexation in search engines are most important factors for website owner or administrator. Search engine crawls only static contents and if you have dynamic links like javascript, crawler will not take efforts to extract links. Another thing is suppose you don’t want to put some URLs (say A) on your homepage or on the pages that are directly or indirectly linked to homepage in this case when crawling takes places search engine will not get navigated to these links (A). In this case also XML sitemap plays important role.

So XML sitemap standard is made of search engines like Yahoo, Google, MSN, Ask and it is the best way to inform SE about the webpages present on your website. Web crawler will easily pickup links and other metadata from it and easily navigate to even dynamic pages.