Linkspam is a manipulation of the link situation of a website to improve its ranking. A link spam or index spam is characterized by the fact that as many backlinks as possible are set to a website. These can be paid, free or acquired in exchange backlinks.
If such links are set as dofollow links and they are thematically not in context with the content of the website, or it is a website from the Bad Neighborhoodcategory, then the website is negatively evaluated and devalued by the search engines. As a result, instead of improving the PageRank, the search engine operator devalues it.
The term Linkspam is used by Google for the setting of backlinks when they accumulate unnaturally. They thus correspond to a black-hat SEO attack. Such websites with a conspicuous number of dofollow links often have little content substance and aim solely at a high ranking.