This is something we see time and time again. It is fair to say, unless you have had a bit of experience with it, can be a bit confusing.
So here is a breakdown of the domain name components..
This is the business you pay a yearly fee to register your domain name. i.e. reserve it for you. i.e. so you 'own' that domain. Often your ISP will provide a registrar service. Or people use services like godaddy , enom, gandi or 1stdomains.
Some Registrars will also be provide your name server/DNS hosting as well. Since this can often make it a bit easier for people.
The name server/DNS server is where all your DNS records are stored. It is how a domain name gets converted into an IP address. It is where servers figure out where to deliver email for your domain.
The registrar is the authority for which name servers to use for your domains. You tell them what name servers to use. And they let the world know (by putting that information into the 'root' DNS servers).
There can be multiple Hosts or just the one. Usually one for email, another can be website. A lot of hosts do both from the one server if they are smaller, and larger places such as an ISP will have multiple mailserver, multiple webservers etc.
How RimuHosting Can Help
RimuHosting do not provide registrar services. We do provide a DNS server, and we do, of course, provide hosting services.
To use our DNS servers, setup a DNS 'zone' at http://rimuhosting.com/dns. Point that at your servers with us. Then change your name servers at your registrar to the RimuHosting name servers (which we will list after you have setup your zone with us).
You can read a bit more about our DNS service at http://rimuhosting.com/dns/aboutdns.jsp