Cost free advertising is where either the service is provided totally free or the is some sort of exchange of services. In most cases these networks provided a system where by site owners add a unique code to there site. The code rotates banners and text adverts, these adverts are added to the system by other website owners. Often you earn points for each time an advert is shown on your site. These points are then used for you to show your own adverts in the network. Some networks are like for like, other keep back some advert slots and fill them with affiliate adverts which pays them for providing the exchange service.