Domain, domain address, primary domain or domain name are different names for the same thing.
Examples of domain names are mumbaihosting.com, ports.se, tuborg.dk, rover.co.uk, privatpersonnamn.pp.se, usr.ru, passagen.se, and aftonbladet.se etc. You could say that it is a character designation for a group of computers on the Internet.
A domain consists of several parts. If we take the domain aero.volvo.se for example, it consists of three parts. (www.aero.volvo.se is the computer in the domain aero.volvo.se, as explained above).
Taking it backwards, se is the top-level domain or primary domain for Sweden (often written TLD – Top-level Domain) and all the subdomains under it.
(volvo is the subdomain to se and the top-level domain for the company Volvo, aero is the subdomain to the top-level domain volvo).
In other words, the dot always separates the subdomains and the top-level domain, which is also the reason why dots can never be used in the domain name.
Subdomains never need to be registered, other than with the party that has registered the top domain (volvo.se is registered with se, and the domain aero.volvo.se with Volvo).
You could conceive of even more subdomains in the above example, such as goteborg.finance.aero.volvo.se, which could then possibly be the domain for the Gothenburg Division of Volvo Aero Finance.
The domain in the above example – aero.volvo.se – actually consists of four parts.
There is actually a top-level domain called “.” (dot nothing), which signifies the whole Internet and is the top-level domain for all top-level domains. So to be totally correct, we should write aero.volvo.se. – that is with a dot at the end.
Usually you do not write it, since it is somewhat confusing as it is with all the other dots, but it could further our understanding to know that se is the subdomain to “.” (dot nothing).