Registration requires the use of a name server or host for each domain registered. Parking is the process by which one chooses a domain name, and "parks" it by registering the domain name under one's name server. Parking can be done by anyone, to anyone who has an active name server. However, parking a domain name alone will not generate services (webhosting, email) for that particular domain name.
In other words, a parked domain is an additional domain that you host on your hosting account. It will display the same website as your main domain and NOT have separate web statistics, but you can give the parked domain its own email box. This is often used by businesses that want to have more than one web address for advertising purposes.