Search engines use links to discover the Webpages, and counts the links as votes for those pages. Now it is good to know that how this information is used by the search engine once they acquire it.
Search engines need to decide which pages to include in their indexes. One way by which search engine discovers pages is by following links and another is through the use of XML sitemap file. Search engines do not index the pages that they consider to be of very low value, a quality link value is the factor used in making that decision.
Search engine spiders crawl a portion of the web everyday; it starts with deciding from where to begin and where to go. Google has publicly stated that it starts its crawl in reverse pagerank order, higher the pagerank, higher the site will be crawled deeply. One of the Google’s update called “caffeine” greatly increased crawling capacity and speed.
Links play important role in ranking, for two similar websites, links work as a voting factor, the site which has more links from important websites wins at last.