Domain names are organized in subordinate levels (subdomains) of the DNS root domain, which is nameless.
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- wikimedia
The DNS root domain is unnamed which is expressed by having an empty label in the DNS hierarchy, resulting in a fully qualified domain name ending with the top-level domain. However, in some cases the full stop (period) character is required at the end of the fully qualified domain name.
The DNS root is essentially equivalent to the DNS root zone, which is served by thirteen root server clusters which are authoritative for queries to the top-level domains of the Internet.
Thus, every name resolution either starts with a query to a root server or uses information that was once obtained from a root server, which is therefore essential for the functioning of the entire DNS system - wikipedia 
The first-level set of domain names are the top-level domains (TLDs), including the generic top-level domains (gTLDs), such as the prominent domains .com, .info, .net, .edu, and .org, and the country code top-level domains (ccTLDs).
# See also