A domain name is a website URL, is the address, is your website name where Internet users can find and visit your website. Computers use IP addresses, which are a series of number. However, it is difficult for people to remember. Because of this, domain names were developed and used to identify entities on the Internet rather than using IP addresses.
A domain name can be any combination of letters and numbers, and it can be used in combination of the various extensions, such as .com, .net and more.
The domain name must be registered before you can use it. Every domain name is unique. If someone types in www.yourdomain.com, it will and only go to your website.
Domain names are formed by the rules and procedures of the Domain Name System (DNS). Any name registered in the DNS is a domain name.
"Domain names are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless. The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains: com, info, net, edu, and org, and the country code top-level domains (ccTLDs). Below these top-level domains in the DNS hierarchy are the second-level and third-level domain names that are typically open for reservation by end-users." - Wiki
The top-level domains (TLDs) such as com, net and org are the highest level. Top-level domains form the DNS root zone of the hierarchical Domain Name System. Every domain name ends with a top-level domain label.
Below the top-level domains in the domain name hierarchy are the second-level domain (SLD) names. These are the names directly to the left of .com, .net, and the other top-level domains. As an example, in the domain sample.co.us, co is the second-level domain.
Next are third-level domains, which are written immediately to the left of a second-level domain. There can be fourth- and fifth-level domains, and so on, with virtually no limitation. Each label is separated by a full stop (dot).