Back in the late 90’s, when search engines like Excite, Altavista, Hotbot and Lycos ruled the world, search results were determined only by relevancy.The more keywords on your page that matched the query, the higher your page would rank.

Then Google flipped the search world upside down with its notion of PageRank. The link based popularity metric that introduced the concept of authority + relevancy as its ranking mode.

PageRank Rules the Day:

Google was 100% right in thinking that authority, or credibility, should be a ranking factor, the problem was that Google had no way of telling how authoritative the author of the page really was. In fact, in most cases they couldn’t even tell “Who” the author of the page was. The best they could do was to look at what other domains linked to them and from that try to calculate a popularity score – PageRank.