For e.g. yourdomain.com -> mydomain.com i.e. yourdomain.com and mydomain.com will open same website. The main domain mydomain.com will need both web hosting and domain registration. Whereas, parked domain yourdomain.com will only need domain registration.
In addition to this, nameservers (DNS) for both domains must be same. This is very important unless, parked domain will not work. To obtain namesevers for your domain, please contact to firstname.lastname@example.org.