The hierarchy of the domain is as follows:
A domain is a sequence of labels from the node at the root of the domain to the root of the whole tree, with the labels separated by dots.
Each DNS has TLD i.e. Top level domain define like .com or .org. or .in. They have got the authority for naming of the DNS. Under this we have domain name like here we have example domain. Each of the top-level domains has a number of name servers. These are the servers that have information on the hosts within the domain. Under top level domain we have got subdomain like us.exmple.com here us is the subdomain of the example.com. Under this we have individual host name that exist.