The search engines use links primarily to discover web pages, and to count the links as votes for those web pages. But how do they use this information once they acquire it? Let’s take a look:
Index inclusion : Search engines need to decide which pages to include in their index. Discovering pages by crawling the Web (following links) is one way they discover web pages (another is through the use of XML Sitemap files). In addition, the search engines do not include pages that they deem to be of very low value because cluttering their index with those pages will not lead to a good experience for their users. The cumulative link value, or link authority, of a page is a factor in making that decision.
Crawl rate/frequency : Search engine spiders go out and crawl a portion of the Web every day. This is no small task, and it starts with deciding where to begin and where to go. Google has publicly indicated that it starts its crawl in reverse PageRank order. In other words, it crawls PageRank 10 sites first, PageRank 9 sites next, and so on. Higher PageRank sites also get crawled more deeply than other sites. It is likely that Bing starts their crawl with the most important sites first as well.
This would make sense, because changes on the most important sites are the ones the search engines want to discover first. In addition, if a very important site links to a new resource for the first time, the search engines tend to place a lot of trust in that link and want to factor the new link (vote) into their algorithms quickly. In June 2010 Google released an update to their infrastructure that they called Caffeine which greatly increased their crawling capacity and speed (http://googleblog.blogspot.com/2010/06/our-new-search-index-caffeine.html), but being higher in the crawl priority queue still matters.
Ranking : Links play a critical role in ranking. For example, consider two sites where the on-page content is equally relevant to a given topic. Perhaps they are the shopping sites Amazon.com and (the less popular) JoesShoppingSite.com (not a real site).
The search engine needs a way to decide who comes out on top: Amazon or Joe. This is where links come in. Links help cast the deciding vote. If more sites, and more important sites, link to Amazon, it must be more important, so Amazon is more likely to rank higher.
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