A linking scheme is any links intended to manipulate Google's PageRank or TrustRank algorithms. Link schemes (years ago) could elevate a website in Google's SERPs; such schemes included buying links on many, many websites having nothing to do with your site or services or products. These kinds of links used to be sold by link brokers. It was also possible to get links from many directories. One could link out to other websites in exchange. Link Schemes are Bad and will get you penalized at Google. Don't do this.