Every single registered domain name has at least two Name Server records that show where it is hosted i.e. by using these records you point your domain to the servers of a particular hosting company. This way, you have both your website and your e-mails handled by the same provider. On the lower level of the Domain Name System (DNS), however, there are lots of other records, like A and MX. The first one shows which server deals with the site for a given Internet domain and is always an IP address (18.104.22.168), while the second one shows which server deals with the emails and is always an alphanumeric string (mx1.domain.com). As an example, whenever you type a domain in your web browser, your request is forwarded through the global DNS system to the company whose NS records the domain uses and from there you could be forwarded to the servers of a different 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 you may have your website and your emails with two different providers if you would like.