Often, the company can register your domain name for you. If you have a domain name, you will need to get the DNS information from the URL redirection service. Check with the particular service for details. Well, they do with the paid service. But I don't know why not with the free accounts, other then that the URL for your account is basically the only thing that lets someone know what company is providing the web hosting service.
Domain parking is basically what it sounds like.
Domain parking lets you register your domain name and park that domain name at ProHosting. The main reason is so you can get that domain name registered now while it's still available, but before you're ready to actually use it. While ProHosting does offer free domain parking, this service has nothing to do with the free or the paid web hosting accounts. When you go to Network Solutions or some other domain registration service, one of the pieces of info you need is the DNS server addresses of or some other domain registration service, one of the pieces of info you need is the DNS server addresses of the place that will be hosting your domain. Without this, you can't register the domain.