The use of link analysis by internet search engines to determine relevance resulted in an effort to manipulate such link analysis systems.
Putting a link farms link at the bottom of every page in a site. The links in the farm point to every other page in that site, or to another site controlled by the author. Link farms are easy to spot, hence pseudo web-rings and random linkage within a member group appeared. In February 2011, Google launched Google Panda, intended to improve its spam-detection algorithm.
Doorway pages consist entirely of links and are not intended to be viewed by humans, but are made just to be discovered by search engines. These pages can have thousands of links, including multiple links to the same object.
Link farms and doorway pages are effective when the link analysis is sensitive to the absolute number of links. Ranking techniques using quality of links are not that vulnerable to these spam techniques.
A common form of link spam is the use of link-building software to automate SEO processes.
Private blog networks are groups of authoritative websites used as a source of contextual links that point to a particular site to achieve higher search engine ranking for it. This strategy buys up expired domains or auction domains that have backlinks from high-authority websites. Search engines responded with re-indexing campaigns.
Indirect link spamming practices increased after the campaigns against link farms and private blog networks.