A domain must be unique (for example there can only be one single Yahoo.com) and must be registered with a domain registrar (ie. NameCheap); while for sub domains, users can freely add it on top of the existing domain as long as their web host provide the service. Some would say sub-domains are the ‘third level’ domains in the sense that they are simply “sub folders” under the domain root directory, normally used to organize your website content in different languages or different categories.

A domain name is a unique web address on the Internet, like www.example.com. Much like buying real estate in the physical world, when you register a domain name it becomes your property in the online world. No two domain names can be the same which means that if the domain name you want is already registered, you are unfortunately out of luck. Domain names can also have different domain extensions, or TLDs, attached to the end of it. For example, you may own www.example.com, however www.example.net may still be available.

For special purposes, such as network testing, documentation, and other applications, IANA also reserves a set of special-use domain names.[15] This list contains domain names such as example, local, localhost, and test. Other top-level domain names containing trade marks are registered for corporate use. Cases include brands such as BMW, Google, and Canon.[16]

The U.S. Congress passed the Combating Online Infringement and Counterfeits Act in 2010. Consumer Electronics Association vice president Michael Petricone was worried that seizure was a blunt instrument that could harm legitimate businesses.[29][30] After a joint operation in February 15, 2011, the DOJ and the Department of Homeland Security claimed to have seized ten domains of websites involved in advertising and distributing child pornography, but also mistakenly seized the domain name of a large DNS provider, temporarily replacing 84,000 websites with seizure notices.[31]
For those users who are seeking to register a country-specific domain name option (like “.us” or “.co.uk”), a good portion of the registration process will be dedicated to determining whether or not the customer is a resident of that country and therefore legally permitted to purchase one of its country-specific top level domains (will talk about this later). And that should hammer home a secondary point to users.

For example, only organizations can register a “.org” domain name, and only American citizens can register a domain name that ends in “.us.” Failing to meet the guidelines and requirements for each type of domain during the actual registration and payment process will result in the domain name being “released” back into the pool of available domain names; the customer will have to pick a top level domain for which they actually qualify, or cancel their purchase altogether.
Moving to another website consists of transferring the website’s files and databases, configuring your site with the new host, and directing your domain’s DNS to the new host. Once you pick a new site host, they can usually help you out with this process. The cost will depend on the host you’re switching to, but it will probably be anywhere from $150-$400.