Google has been dominating the world of search engines since decades. In the year 2000 and till 2003, search engines were like keyword-matching and link-counting machines and there was simple 2 concepts to rank high in SERPs. Either stuff your keyword phrase into your blog/website as much as possible or get a lot of inbound links to your blog/website. Things have changed with times and Google also keeps changing its algorithm for which a lot of SEO experts who find comfort in dealing with SEO practices face challenges to rethink in a different way.
Quality content and backlinks have always been the major factors for a website's ranking. Google has always focused mainly on these two factors and introduced several other factors too. Lets see in the below infographic when and why Google changed its algorithm.
Source : HubSpot