Resolvers are the clients that access name servers. Programs running on a host that need information from the domain namespace use the resolver. The resolver handles:
Querying a name server
Interpreting responses (which may be resource records or an error)
Returning the information to the programs that requested it
In Windows 2000, the resolver is just a set of library routines that is linked into programs such as telnet and ftp. It’s not even a separate process. It has the smarts to put together a query, to send it and wait for an answer, and to resend the query if it isn’t answered, but that’s about all. Most of the burden of finding an answer to the query is placed on the name server. The DNS specs call this kind of resolver a stub resolver.
Other implementations of DNS have had smarter resolvers that can do more sophisticated things, such as build up a cache of information already retrieved from name servers. In fact, Windows 2000 includes a caching resolver.