Subdomain is a domain that is part of a larger domain name in Domain Name System (DNS) hierarchy. A subdomain is a domain that is part of a larger domain. In DNS hierarchy consists of the root-level domain at the top, below that are the top-level domains, followed by second-level domains and finally subdomains. A subdomain expresses relative dependence, not absolute dependence.
How to create a subdomain:
Subdomains are commonly used by organizations that wish to assign a unique name to a particular department, function, or service related to the organization. A subdomain is sometimes termed a vanity domain, can be a subdomain of an ISP's own domain also known as to an individual user account or expresses the individuality of the person on whose behalf it is registered.