Looking back over the 15 or so years that SEO has been researched, discussed and practiced, it’s difficult to find any significant period of time when it wasn’t changing.
Before Google came onto the search scene around the turn of the millennium, the search engines at the time were ranking websites based primarily on the sheer volume and density of keywords on the page. So ranking for “blue widgets” meant little more than finding ways to stuff those two words into every inch of your page possible while still maintaining at least the semblance of a user experience. The result was a plethora of webmasters who found creative ways to stuff keywords into every corner of their sites. Then Google changed the game.
With PageRank, Google introduced a new way of evaluating the relative authority of a website or page - links. PageRank, to put it in simple terms, provided a link map of the web. The more links pointing to a site or page, the more power or authority that page took on. And the anchor text of a link, the words that occur as clickable text, offered a clue as to what the linked-to page was all about. As Google garnered more market share, links became more valuable on the web. A whole new form of webspam was born - free-for-all links and link selling schemes. Google has yet to find an effective way to completely crack down on these tactics, but in the 3rd quarter of 2007 they took some signficant steps including directly penalizing specific websites which were believed to be selling links for SEO benefits and relieving “free-for-all” directories of their power to pass PageRank altogether.