A domain or domain name is “the name” that identifies a website which would be your trademark. Each domain has to be unique on the Internet. For example, “www.starclownonline.com” is my domain name which is my “brand” and no one else can “register” the same name as a domain name for their website because I have purchased that right and I pay for it every year.
You can choose to buy the right domain for a year, two or five years if you want. Utility companies offer generous discounts if you do it for longer.
Another thing you should know is that a single server can serve multiple websites and multiple domains, but a domain can only point to one server.
A domain is usually composed of three parts, eg. www.starclownonline.com, the three “W” (www) meaning: World Wide Web; the name of the organization (starclownonline) and the type of organization (.com). These days you will find a lot of “extensions” like .club, .online, .fashion, etc.
The most common types of organization are .com, .net, .mil and .org, which relate to commercial, network, military, and organization originally created to entities that establish a non-profit organization, but now anyone can register a .org domain.
Since the Internet is based on IP addresses, not domain names, every Web server requires a Domain Name Server (DNS) translate the domain names to IP addresses. Each domain has a primary name server and secondary domain.