Every registered domain has no less than two Name Server records which show where it's hosted i.e. by using these records you direct your domain name to the servers of a certain web hosting provider. In this way, you have both your site and your e-mails handled by the exact same company. On the lower level of the Domain Name System (DNS), on the other hand, there are lots of other records, like A and MX. The former reveals which server deals with the website for a given domain and is always an IP address (18.104.22.168), while the latter shows which server handles the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you enter a domain in your web browser, your request is forwarded through the global DNS system to the company whose NS records the domain address uses and from there you will be sent to the servers of another service provider provided you have set an IP address of the latter as an A record for your domain address. Having independent records for the website and the e-mails means that you may have your site and your emails with 2 different providers if you'd like.