Registries require the use of name servers or hosts for every domain registered. Parking is the process by which someone selects a domain name, and "parks" it by registering the domain name under someone's name servers. Parking can be done by anyone, to anyone else who has active name servers. However, parking a domain name alone will result in no service (webhosting, e-mail) for that particular domain name.
In other words, parked domains are additional domains that you host on your hosting account. It will display the same website as your main domain and do NOT have separate web statistics, but you can give the parked domain its own email boxes. It's often used by businesses that wants to have more than one web address for advertising purposes.