Cache

Resolution result can be cached and reused
improve response time, decrease the load of upper level authoritative servers

How long is the cached data valid?
until the TTL of the RR expires.
TTL=3600 => the cached RR expires in an hour.

Caching server (aka recursive resolver)
concentrating on name resolution and caching
(conceptually) do not have an authority of any zone
novice users call a caching server a "DNS server"
they use 'stub resolvers' which ask caching servers.