When you register a domain name, you register what is known as the root domain, also called naked domain or the zone apex. There are only two parts, your preferred name, and the extension. So for chocolateyfish.com, for example, "chocolateyfish" is the name you've chosen, and ".com" is the extension. Together, chocolateyfish.com is a root domain.
There are two kinds of extensions, gTLD (general top-level domain), and ccTLD (country code top-level domain). gTLDs include many of the most common types of extension, like .com or .net, as well as the new or nTLDs, like .photos or .pizza. ccTLDs are all the two-letter country code extensions, like .ca, .de, or .nz.
The governing body ICANN is in charge of the rules and policies for all gTLD extensions. However, each country's individual registry makes its own rules and policies for each ccTLD. This is why you may find those rules and requirements vary a lot.
For chocolateyfish.com, there's nothing at the beginning before "chocolatey", like "www" or "blog" or "shop". Adding any one of those is called a subdomain. So while domains with www, like www.chocolateyfish.com are more common than shop.chocolatelyfish.com, they are all subdomains. So you can't register a subdomain, including with www. You can only register a root domain.
Technically, domains are read from right to left, which can seem backwards.
- The domain starts with a period ".", which is usually not displayed.
- Next is the extension, or top-level domain, e.g. .COM or .NZ.
- That is followed by the root domain, which is what you can register, and which is assigned to you, e.g. chocolateyfish.com.
- Anything after the root domain is a subdomain. You can create as many subdomains for your domain name as you like, and up to the maximum allowed length of a domain name, which is 253 characters in total. However, each 'level' may not be longer than 63 characters. E.g., with "mydomain.com" something like "this.is.a.subdomain.of.mydomain.com" could be created.