Google uses several hundred ranking factors in their search algorithm.
PageRank works by counting the number and quality of links to a page to determine a rough estimate of how important the website is.
The underlying assumption is that more important websites are likely to receive more links from other websites.
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FAQ: All About The New Google RankBrain Algorithm
RankBrain is part of Google’s overall search “algorithm,” a computer program that’s used to sort through the billions of pages it knows about and find the ones deemed most relevant for particular queries.