Each registered domain has a minimum of 2 Name Server records which show where it's hosted i.e. by using these records you direct your domain address to the servers of a particular web hosting company. That way, you have got both your site and your e-mails handled by the very same provider. On the lower level of the Domain Name System (DNS), on the other hand, there are lots of other records, for instance A and MX. The first one shows which server handles the website for a given domain name and is always an IP address (18.104.22.168), while the latter reveals which server deals with the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, whenever you enter a domain address in your web browser, your request is forwarded 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 in case 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 emails suggests that you could have your site and your emails with two different companies if you wish.