Comment spam is a black hat SEO practice which consists of posting high numbers of comments on news websites or blogs for the sole purpose of gaining many backlinks and improving google’s SERP positions.
Comment spam can be manual or automated through dedicated sofwares.
To be effective and taken into account by Google and other search engines, comment spam has to be done on dofollow comment spaces. Dofollow comment spaces are localized through URL scraping.
Comment spam is becoming less and less effective as Google and others are more and more considering and evaluating backlink quality. Coarsely used it may provoke backlash from Google which could detect too many low quality backlinks.
Comment spam can also be used to try to gain trafic to "suspicious" website.
An example of comment spam :