Each registered domain has a minimum of 2 Name Server records which show where it is hosted i.e. by using these records you direct your domain to the servers of a certain hosting provider. This way, you have got both your site and your e-mails handled by the exact same service provider. On the lower level of the Domain Name System (DNS), however, there are lots of other records, for instance A and MX. The first one shows which server handles the website for a given domain and is always an IP address (18.104.22.168), while the second one reveals which server manages the e-mails and is always an alphanumeric string (mx1.domain.com). For instance, any time you type a domain name in your web browser, your request is sent through the global DNS system to the provider whose NS records the domain name uses and from there you will be directed to the servers of a different service provider provided you have set an IP address of the latter as an A record for your domain name. Having independent records for the website and the e-mails means you can have your website and your e-mails with two different companies if you'd like.