While there are other nameserver roles (such as forwarders, which sit inside internal networks and pull queries from the outside world), we've seen that there are two kinds of nameservers that correspond to the two sides of the DNS query: a question (resolver) and a response (authoritative).
Resolvers are the ones asking the questions. They may be separate servers or they may be an onboard application. There are privacy implications of public resolvers. (I personally expect a growing trend toward personal resolvers on one's own device or under one's control, but I could easily be wrong. These days, people seem content to post their entire lives on Facebook, so maybe nobody will care if every single website, address, or hostname they look up turns out to be vacuumed, logged, cross-referenced, analyzed, repackaged, retargeted, rehypothecated, and sold.)
Authoritative...
