Getting other good quality, relevant websites to link to your content can help improve your website’s visibility in the search engine results. The incoming links, or “backlinks” should be from websites that are relevant to yours in subject matter, and ideally they should be reputable, good quality sites.
Getting the links that will actually help is no easy task, and some webmasters and SEOs resort to buying links from other websites. The use of such paid links that pass “PageRank” through to the target site are a violation of Google’s Webmaster Guidelines and can cause a website to rank much lower due to a penalty for “unnatural links“. There are exceptions to this rule, like certain directories where the money paid to the directory site is not exactly for the link, but for the time it takes someone to review your site to be sure it is what you say it is, or for additional directory features like better positioning within the directory listings. Be careful when submitting your website to any directories which require a payment as this is one area where Google is not extremely clear
As far as Google is concerned, paid links could also include things like product reviews where the reviewer received free merchandise in exchange for the review and link.