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The classic parking model is to point a domain towards a system that throws up ads automatically, on the first loading of the domain.
The next version is the '2 click lander'. The first lander that loads, when someone types the domain name into a browser, offers a selection of keyword links that, when clicked, load a second page keyword-link targeted ads.
There are emerging versions that integrated a level of content onto the landing pages, making for more of a 'website appearance'.
There are millions of domains that are working with automated content and automated advertising systems of various degrees of sophisitication. The success of any domain is a function of its level of type-in traffic it receives, the nation of origin of that traffic, the availability of advertisers targeting that traffic, the PPC rates they are willing to pay, the market focus of the domain (kids, jokes, commercial), the system the domain is working with, the number of domains and level of traffic the domainer brings to the system (impact on revenue sharing), and other factors.
Parking is a nice interim measure for those who have a portfolio of domains that they intend to develop as time, money and opportunity allows. If you pick the right domain you can begin to recoup at least the registration fee pending development. If you pick with great success or skill you can do a lot better.
Focus on .com. After that focus on very robust .net, .org or, in rare cases, .info.
Don't go overboard. Picking the right domains takes a good deal of insight, experience, patience, reserve, etc.