Text Link Ads: A text link ad is text on a website or blog that contains a hyperlink to a specific page on a different website. The individual who runs the blog or website, who is referred to as the publisher, is paid by an advertiser who's using their site to drive traffic to the page in the link.
The pay publishers receive from this advertising technique consists of either a flat fee or pay-per-click, which generates revenue for the site for every time the link is clicked.
Text link ads have caused controversy in recent years because sites that feature them (and sites that pay for them) often see a drop in Google search rankings, or are eliminated from Google search results altogether.
Some publishers have tried to use HTML No-Follow tags in text link ads to disguise them from Google search spiders, but many advertisers don't approve this practice. Major link seller Text Link Ads took a major hit from Google's attempts to curb spam linking in 2013.
Examples of popular text link advertising program's include Kontera and Google's own AdSense, which both offer contextual text link advertising. This means that rather than targeting random words, linked ads will only appear in relevant content.