Each registered domain name has at least two Name Server records that show where it's hosted i.e. by using these records you direct your domain name to the servers of a particular hosting company. That way, you have got both your website and your emails managed by the same company. On the lower level of the Domain Name System (DNS), on the other hand, there are lots of other records, for instance A and MX. The first one reveals which server handles the website for a given domain address and is always an IP address (18.104.22.168), while the latter indicates which server handles the emails and is always an alphanumeric string (mx1.domain.com). As an example, when you type a domain address in your browser, your request is forwarded through the global DNS system to the company whose NS records the domain address uses and from there you could be sent to the servers of another company in case you have set an IP address of the latter as an A record for your domain. Having separate records for the website and the e-mails means that you could have your site and your e-mails with 2 different providers if you'd like.