Search engine optimization (SEO) is a technique which helps search engines find and rank your site higher than the millions of other sites in response to a search query and SEO will help you get the high traffic.
This SEO tutorial covers all the necessary information you need to know about Search Engine Optimization – what is it, how does it work and differences in the ranking criteria of major search engines.
Search engines will crawl the Web to see what is there. This task is performed by a piece of software, called a crawler or Googlebot (as is the case with Google). A Spiders will follow links from one page to another and index everything they find on their way. Having in mind the number of pages on the Web (over 20 billion), it is impossible for a spider to visit a site daily just to see if a new page has appeared or if an existing page has been modified, sometimes crawlers may not end up visiting your site for a month or two.
source : webconfs