Ask anyone if they know what the search engines are looking for when they analyse a website, and 90% of the answers you’ll get will be a shrug of the shoulders. Those with a knowledge of SEO will give you better answers but they can vary between the oversimplified to the downright confusing, and everything in between.
Other than the search engines themselves, nobody can be 100% sure of every single element that the search engines look at, how it assesses them, and the importance it places upon them. To be fair to the search engines they do publish information that gives webmasters and businesses some of the core answers they need to ensure any website they are using is as SEO friendly as it can be.
There is also a lot of research that takes place, mainly by SEO agencies who create hundreds of websites, in order to analyse what happens to their ranking following specific changes which are made to that site. The information given out by the search engines plus the published research has established there are four basic elements which contribute to the ranking of any website.