Each of these services has a different purpose and is billed differently. Together, these three components form the chain that links a domain name (like example.com) to a web site (like example.nfshost.com).
Domain Registration is the process of reserving a domain name with a central authority (called a registry), so that no one else but you can use it.
Domain names are of the form example.com as opposed to host names (aka site aliases), which tend to be of the form www.example.com. Domain registrars act as intermediaries between you and the registry, and they charge a fixed annual fee for this service. We provide domain registration services for the most common top level domains.
DNS or Domain Name Service
DNS or Domain Name Service is used to tell the world what you want to do with your domain name. Most commonly, it is used to create a hostname.
Most hostnames have another word attached to the domain name like www.example.com. Each hostname corresponds to a website. It is possible to have more than one hostname per website, but a hostname can only refer to a single site or IP. DNS links this hostname to your website, so when people want to visit www.example.com, their web browser can find it. This is a little like calling directory assistance to have them find a phone number based on someone's name.