Every registered domain name has no less than 2 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 particular website hosting provider. In this way, you have got both your website and your e-mails managed by the same provider. On the lower level of the Domain Name System (DNS), however, there are lots of other records, for example A and MX. The first one reveals which server deals with the website for a given domain address and is always an IP address (18.104.22.168), while the second one indicates which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). For instance, when you enter a domain in your browser, your request is sent through the global DNS system to the provider whose NS records the domain uses and from there you may be forwarded to the servers of a different provider if you have set an IP address of the latter as an A record for your domain address. Having separate records for the site and the e-mails means you could have your site and your e-mails with two different providers if you'd like.