Paid links are links that people place on their site for a fee, believing that this will have a positive impact on the search results. The practice of paid links was very popular prior to the Penguin update, when companies believed they could add any types of links with impunity, since Google claimed prior that they ignored these links instead of penalizing websites. To comply with Google's recent TOS, it is imperative to apply the nofollow attribute to paid advertisement links. Businesses that buy backlinks from low quality sites attract Google penalty.
