Every single registered domain has at least two Name Server records which show where it's hosted i.e. by using these records you direct your Internet domain to the servers of a certain hosting company. That way, you've got both your site and your emails handled by the very same service provider. On the lower level of the Domain Name System (DNS), on the other hand, there are a number of other records, like A and MX. The former shows which server deals with the site for a given domain name 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). As an example, when you enter a domain in your Internet browser, your request is sent through the global DNS system to the provider whose NS records the domain address uses and from there you will be forwarded to the servers of a different service provider in case 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 may have your site and your e-mails with 2 different providers if you'd like.