Every single registered domain has a minimum of 2 Name Server records which show where it is hosted i.e. by using these records you point your Internet domain to the servers of a particular hosting company. That way, you have both your website and your emails handled by the same provider. On the lower level of the Domain Name System (DNS), on the other hand, there are lots of other records, for example A and MX. The former reveals which server manages the website for a given domain name and is always an IP address (18.104.22.168), while the latter reveals which server manages the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time you enter a domain address in your Internet browser, your request is directed through the global DNS system to the provider whose NS records the domain name uses and from there you could be directed to the servers of a different provider if you have set an IP address of the latter as an A record for your domain name. Having separate records for the site and the e-mails means that you could have your site and your e-mails with two different companies if you wish.