Domain registrar vs domain name servers
When you purchase a domain name from a registrar, you are buying a top level registration so that the domain name you purchased has a SOA ( start of authority ) record. This record tells requesters where the nameservers are for your domain, it does not say where the actual domain is hosted or what hosting provider it is under. From your registrar you edit your SOA record when you set your nameservers. Some registrars also provide DNS services for you with an additional fee.
The nameservers themselves tell the requester where the domain name lives for hosting. There are multiple records that existing with your name server provider, the most important being the A record for your domain. The A record ( stands for Address record ) directly maps to an ip address for your domain name. This ip address tells the requesting client program where the request needs to go. Other common records on your name servers include: your email server ( usually mail.yourdomain.com ), your MX record ( mail exchange, tells email servers what domain to use when sending email to email@example.com ), and a CNAME record for www ( stands for Canonical NAME, used for subdomains that point to the same place as the primary A record ).