Each registered domain has at least two Name Server records which show where it is hosted i.e. by using these records you point your domain address to the servers of a certain hosting company. In this way, you have both your website and your e-mails managed by the same service provider. On the lower level of the Domain Name System (DNS), however, there is a variety of other records, such as A and MX. The former shows which server deals with the website for a given domain and is always an IP address (18.104.22.168), while the second one reveals which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, when you enter a domain in your browser, your request is directed 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 another service provider provided you have set an IP address of the latter as an A record for your domain. Having independent records for the website and the emails means you may have your site and your emails with 2 different companies if you would like.