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 name to the servers of a particular hosting provider. In this way, you have both your website and your e-mails managed by the exact same provider. On the lower level of the Domain Name System (DNS), on the other hand, there are lots of other records, such as A and MX. The former shows 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 reveals which server deals with the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, when you enter a domain name in your web browser, your request is directed through the global DNS system to the company whose NS records the domain uses and from there you will be sent to the servers of a different company if you have set an IP address of the latter as an A record for your domain address. Having separate records for the website and the e-mails means that you can have your website and your emails with 2 different providers if you would like.