We all know how important links are; the lifeblood of just about any SEO campaign. The more links point to a particular URL, the more important that page is assumed to be. Like any rule of thumb, that's an over-simplification, but it's true enough that if you only know a few things about search engine optimization, that needs to be one of them.
Further, the anchor text of these links is assumed to describe what a page is generally about; if you want to rank for a particular phrase in a competitive landscape, you need help from off-page optimization. Out of about 150 million results, Adobe's Acrobat Reader ranks #1 for "click here" even though the text is nowhere on the page.
From Humble Beginnings
The relationship attribute of a hyperlink ( a element ) was given a new, and by now widely recognized value, nofollow, in January 2005. This new rel="nofollow" code in a link tells the major search engines not to count the link it's applied to in their ranking algorithms. Google introduced this new value to help bloggers deal with comment spam; by removing any SEO benefit from links on unmoderated sites, to make them less attractive targets.
Wikipedia upped the ante, applying nofollow to all outgoing links from their site, except to a handful of wikia sister projects and internal pages. A high traffic, free encyclopedia that ranks well became a great spam target. Blog spam expanded into wiki spam.
Earlier this year, Google announced that it would like help identifying sites that sell links without using nofollow or some means of hiding the links from GoogleBot. Recent events show they're serious. Naturally, it didn't take long for webmasters to start using nofollow for any number of reasons; there are a few sprinkled into this post to illustrate the point. Can you tell which ones?
Rise of I-Follow or DoFollow
All of the major blogging platforms adopted nofollow in their comment links quickly. How could a change like this not see resistance? A movement calling itself "I-Follow" has sprung up, urging bloggers to remove the attribute from their comment links, and reward visitors who contribute something of value.