As we said, DNS is, at its core, a hierarchical system. At the top of this system is what are known as "root servers". These servers are controlled by various organizations and are delegated authority by ICANN (Internet Corporation for Assigned Names and Numbers).
There are currently 13 root servers in operation. However, as there are an incredible number of names to resolve every minute, each of these servers is actually mirrored. The interesting thing about this set up is that each of the mirrors for a single root server share the same IP address. When requests are made for a certain root server, the request will be routed to the nearest mirror of that root server.
What do these root servers do? Root servers handle requests for information about Top-level domains. So if a request comes in for something a lower-level name server cannot resolve, a query is made to the root server for the domain.
The root servers won't actually know where the domain is hosted. They will, however, be able to direct the requester to the name servers that handle the specifically requested top-level domain.
So if a request for "www.wikipedia.org" is made to the root server, the root server will not find the result in its records. It will check its zone files for a listing that matches "www.wikipedia.org". It will not find one.
It will instead find a record for the "org" TLD and give the requesting entity the address of the name server responsible for "org" addresses.
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