Black Hat technique consisting in the exchange of links on a large scale between a network of websites that link to each other with the sole objective of improving their search engine positioning.
It can also be the purchase of inbound links associated with a desired anchor text in artificial and automated web directories .
In both cases the link comes from conglomerates of thousands of websites without a common theme or logical structure. Do not confuse its practice with the legitimate exchange of links between friendly pages, or thematic directories of websites.
Link farms were created to help increase PageRank, but Google and other search engines have adapted their algorithms and now consider links from link farms, whose impact on positioning has been devalued, to be toxic.