Google divided up most algorithm changes between on-page and off-page. There were content and over-optimization algorithms, and there were link algorithms. The real focus of all of these, however, was spam. Google became the search market leader in part by being better than its competitors at removing irrelevant and “spammy” content from its search results pages.
But today, these algorithms cover so much more. While Google still uses algorithms to remove or demote spam, they are additionally focused on surfacing better user experiences.