Authoritative vs Resolving DNS servers
What is DNS resolver?
DNS Resolvers are responsible for providing the correct IP address of a domain name to the requesting host. For example, if you make a request from your web browser and there is no information on your computer about this domain name (it is not cached), your computer will send the request to DNS resolvers. The resolver will then try to find the name servers, that are responsible for this domain name and contain the necessary records. These name servers are called authoritative.
What is authoritative name server?
Authoritative DNS nameservers provide answers to DNS resolvers with the correct IP addresses and records. These name servers contain the DNS zone with all information, like the IP address of the server, the responsible mail servers, etc for a domain name.
For example, let's say that you would like to open www.cloudns.net. Once you type www.cloudns.net in the address bar and hit Enter, your computer must find the correct IP addresses of ClouDNS web servers. In case it does not have any information about it, your computer will ask the DNS resolvers about the IP address of www.cloudns.net.
First, it will have to ask the authoritative name servers, responsible for the root domain ".", which answer will contain the responsible authoritative name servers for .net. Afterwards the request will continue to the authoritative name servers of .net, as .net is a Top-level domain and there are authoritative name servers for each domain, e.g for .com, .org, .gov, etc. The response will contain which authoritative name servers have information about cloudns.net. Once it got the answer, the DNS resolver will ask the authoritative name servers of cloudns.net, which authoritative name servers are responsible for www.cloudns.net. After a proper response, the DNS resolver will get the IP address of www.cloudns.net and your web browser will connect to it.
Both servers work hand in hand - resolvers ask the authoritative name servers. They cannot be used for different purposes - e.g. authoritative name server cannot be used as DNS resolver or vice versa.