Google was the first search engine to associate search terms with pages by using the link text from the upstream end of a link.

Before that pages were asked to categorize themselves by adding metadata to the HTML source of the page. The title, the headings within the body, and metadata tags in the head were used to categorize for Yahoo, Alta Vista, etc. Google came along and put precedence on the link text. For example in this link: the funniest thing in existence, Google would associate that webpage with the word "funniest" in its search.

By relying on external "categorizations" that relied on the natural English that others used to describe what a link was pointing to, Google was able to tap into a distributed intelligence that associated words with web pages that more closely resembled how a searcher would think of the page.