The purpose of a DNS server is to enable people and applications to lookup records in DNS tables. MostDNS servers are now private, meaning that they are configured to only provide service to the people and organizations who own and maintain them.
A few domainname servers on the Internet provide DNS resolutions for anyone who requests it of them. These are known as "Public DNS Servers."
Most public DNSservers are public on purpose. A few public DNS servers are public only because they have been misconfigured by their system administrators. Those DNS servers tend to eventually be fixed