Because DNS is distributed across domains, when a name server receives a request for name resolution for a host that is outside of its domain, it may not have address information for that host.
Because DNS is hierarchical, it does not need that information, the name server just needs to know how to access the root name server. It forwards the name resolution request to the root name server, which then delegates the request to the appropriate domain beneath it, and this process continues until a name server that has address information for the host is reached, and the information is retrieved.

In order to reduce the length of time of name resolution, and to reduce traffic on the network, important concept of DNS is that of caching.
Whenever a name server receives address information for another host or domain, it stores that information for a specified period of time. That way, if another name resolution request for that host or domain is received, the name server has the address information ready, and does not need to send another inquiry across the Internet.
The length of time address information is stored on the name server is determined by the Time-To-Live (TTL) value entered in the domain Start of Authority (SOA) resource record.