The domains that had been earlier registered to another owner are called as Expired domain. When the previous owner does not pay renewal fees for the domain name, it expires and becomes available in the market.
There is waiting period of around forty-five days before the domain name becomes available in the market. Expired domain name can become major source of profits. There are many automated tools that can be used for identifying and then buying expired domain registration.
There are many expired domain name that have fully functioning web sites along with decent web traffic from links or banner advertisements and through search engine keywords. Therefore, such expired domain names can generate revenue by redirecting to another site or running affiliate programs on them. If the web traffic received by the website is really heavy, then it is possible to even run Google Ad Sense program on the web site. Thus, generate revenues from different sources of online advertising and marketing.