Link schemes are all artificial ways of gaining external links or backlinks to improve ranking in Google and other search engines.
Known and used link schemes are:
creation of many websites, pages, comments linking to the main site
Lines are not always very clear between natural and unnnatural links but Google and other search engines are better and better for detecting link schemes.
In the best case a detected link scheme will not improve rank in SERP and for the worst it will cause a penalty.
Link schemes as defined by Google: