Every registered domain name has at least two 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 certain website hosting provider. That way, you have both your site and your e-mails managed by the very same provider. On the lower level of the Domain Name System (DNS), on the other hand, there is a variety of other records, like A and MX. The first one reveals which server handles the site for a given domain name and is always an IP address (18.104.22.168), while the latter shows which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, whenever you type a domain name in your Internet browser, your request is sent through the global DNS system to the provider whose NS records the domain name uses and from there you will be forwarded to the servers of another company if you have set an IP address of the latter as an A record for your domain address. Having independent records for the site and the e-mails means you can have your site and your e-mails with two different companies if you would like.