Domain Parking is a way to earn money from your domains’ natural traffic. If you have registered domain names, but they are not currently being used, then domain parking is a way to put those domains to work earning you revenue without any work on your part.
It is called ‘parking’ because you point the domain to a provider that takes care of the page that appears to the user when they visit the domain and the advertisements. The parking company does the work for you.
Sedo is a service that provides domain parking and it is free to join.
Essentially the domain is used to display relevant advertisements to the user. Each time a consumer clicks on one of the advertisements you earn money. It is very simple to set up and you can see the reporting provided to watch your domains progress below.
Take a look and give it a try for a couple domains, there are others, Sedo has an excellent easy to use and understand interface.