A DNS server abbreviation means Domain Name System server, it converts a website address into numeric IP address in the process that called recursion, the result let user browser to find server that loads web site main files. The first DNS server your browser checks is our ISP (Internet Service Provider). But with billions of websites, your ISP could not maintain a complete list of each DNS record so it might pass the request to another server If after several attempts DNS servers can’t find a record for the website, you will present a name_not_resolved error. Custom servers like Google dns servers is one of the fastest dns service providers, Cisco OpenDNS, Amazon Route 53 and the market leader Oracle DYN. These providers have hundreds of dedicated domain name servers all over the world, which let them to spread requests and deliver them from the closes server. Extra layer of security and ability to quickly edit any dns record and push the changes to update servers within couple of minutes (comparing to 24 and up to 72 hours at average hosting provider).