DomainParking.com was established in 2003 by a group of domainers and has since become a branch of Traffic Media. The company offers services for services related to parking, appraisal, portfolio management and site building.
The Concept of Domain Parking
Domain Parking is the registration of a domain name without the intent to create and develop a unique site but rather "reserve" it for future development by the registrant or another party. A parked page can potentially generate high levels of traffic, and thus may just be a page full of advertisements.
Many domainers do not do this on their own, but instead contract out their domain or domain portfolio to be parked via a parking company. The company will create the landing page of the website filled with ads and in exchange obtain a percentage of the advertising revenue.  The domainer may be holding onto the parked domain to sell later, or also could be satisfied with retaining the parked page in exchange for consistent advertising revenue.