A sitemap is an XML file that lists URLs for a site along with additional metadata about each URL (when it was last updated, how often it usually changes, plus how important it is, relative to other URLs in the site) so that search engines can more intelligently crawl the site.
The main benefits of a sitemap from the SEO point of view are:
With a sitemap search engines can crawl pages that aren’t otherwise discoverable;
You can give search engines information about a page’s priority with an optional tag in the sitemap. This can help them to order the crawling of the Web site based on priority information;
You can use two more optional tags: “lastmod” tells search engines when a page last changed, and “changefreq” indicates how often the page is likely to change.
If you create your XML sitemap manually, name a file ‘sitemap.xml’ and upload it to the root directory. Any sitemap file that you provide must have no more than 50,000 URLs and must be no larger than 10MB. If you want to list more than 50,000 URLs, you must create multiple Sitemap files and index them in a Sitemap index file.
Search engines are one of the most popular methods to have your target customers find you. If search engines haven’t indexed your important pages through their incoming links, it’s essential that you submit your XML sitemap directly to the search engines.
Tip: If you want your XML sitemap to be indexed more quickly, use your Bing Webmaster Tools account and submit it to Bing manually.