The domain .uk is what is known as a Top Level Domain, or TLD.
Other TLDs amongst many, include .com, .net and .org.
When reading a full domain name or web address, the TLD can always be found at the end of the address, to the right of the dot.
The UK Domain Family also uses what are called Second Level Domains, or SLDs.
This part of the domain address comes directly before the TLD and after the name of the website. For example the org in .org.uk and the co in .co.uk are both SLDs:The UK Domain Family also uses what are called Second Level Domains, or SLDs.
.co.uk – agreatplacetobe.co.uk
.org.uk – nominet.org.uk
A subdomain refers to all the parts of a web address that can be found to the left of the dot; these are the parts of the address that are not the TLD but form part of the web address.
The Domain Name System (DNS) allows for many subdomains in addition to the TLD and SLD.
In practice domain names are not endlessly subdivided. However many organisations use subdomains to identify divisions of that organisation.
For example, the colleges at Oxford University use subdomains: Somerville College’s domain name is some.ox.ac.uk and the University’s main website domain name is ox.ac.uk. Similarly, a company may use subdomains for different brand names.