Subdomain is the domain that is part of another high-level domain.
The structure of DNS (Domain Name System) is a tree and every branch of it is known as a domain name. Such structure consists of mainly subdomains and only root domains are not subdomains of any other domain. For example, ftp.yourdomain.com is a subdomain of the domian yourdomain.com and domain yourdomain.com in its turn is the subdomain of the domain .com, that is known to be a root domain. That’s why it may be said that the connection between subdomain and domain is always a relative one.
Subdomains are most of all used to create unique names for websites for different services or departments of the single organization. Also many companies use subdomains for example to distribute their clients among computing clusters. Also subdomains may be used to define different servers.