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Can any body tell me about Robots and sitemap and how it helps to rank the website

Site maps and robots don't play any positive role in improving search rank, rather they are to tell spiders that which page they should crawl and which they should not.Google normally crawls pages that are linked but if you don't want it to visit any specific page then you can tell this in robot.txt file.Similarly if you think google is not crawling certain page then you can submit sitemap to inform google that how your pages are linked.But usually there is less chance that google misses any page.

The three key processes in delivering search results to you are:

Crawling: Does Google know about your site? Can we find it?

Indexing: Can Google index your site?

Serving: Does the site have good and useful content that is relevant to the user's search?

I think that sitemap is important for ranking because it will make the process of indexing more easier.