As the main aspect to get a higher rank in Search Engine Result Pages(SERP) is considered to be the link popularity that means the number of inbound links to your site to make it popular and place them on to the higher rankings.
The links can be categorized in to two categories, they are:
a. Earned Links
b. Bought Links
Earned Links: The links that are scored with the mutual understanding with the link partners. It may be one-way, two-way, three-way. One-way links are the links in which, there is only a link to your site, no need to link back that very website. Two way links are the links in which the link partners place links to each others site at their link pages. Three way links are those where one links to second site, the second links to third site and the third links to the first site. In all these link exchange programme, all this procedure is performed by the mutual understanding of partners, there is nothing like payment or fees for that.
Bought Links: The links that are bought at some annual charges from some professional websites/directories etc. are said to be Bought Links. In other words advertise of a website on some directories/websites etc. are considered to be the bought links. It is a paid communication through a non-personal medium in which the sponsor is identified and the message is controlled. It may include Publicity, public relations, product placement, sponsorship, underwriting and sales promotion.
Advertising plays a critical role in capitalist economies in creating demand for industrial output. While advertising can be seen as necessary for economic growth, it is not without social costs.