nntpcache (very efficiently) executes on the localhost pretending to be
an NNRP news reading server. In fact, what it does is pass certain NNTP
commands through to real (remote and possibly local) news-servers based
on various pattern matching rules. nntpcache then takes the output from
those servers and caches & indexes it in funky ways (much specific case
magic goes into this).  The next time such information is asked for, or
other information which can be logically inferred from the previously
collated information, it is sent directly from the cache, without
consulting the remote servers.