The two main variants of nameservers represent both sides of a DNS lookup process: a query (resolver) and a response (authoritative nameserver).
Remember my original "elevator pitch" about how DNS works:
"Every time you send an email, visit a web page, type or receive an instant message, text, or SMS, place a VoIP call (or Skype), or anything else involving the internet, it cannot happen until a bunch of computers around the internet have a conversation about it: Where does this email need to be delivered to? What server is holding the file that this web browser is asking for? Where is the VoIP gateway that needs to route this call?"
The computers, or applications or daemons that are actually asking these questions are the resolvers.
Your clients are more often applications such as web browsers or email clients, than they are people. Those...