Every single registered domain has no less than two Name Server records that show where it's hosted i.e. by using these records you direct your domain name to the servers of a particular web hosting provider. This way, you have got both your website and your e-mails handled by the very same provider. On the lower level of the Domain Name System (DNS), however, there are lots of other records, such as A and MX. The first one reveals which server deals with the site for a given domain address and is always an IP address (18.104.22.168), while the second one indicates which server manages the e-mails and is always an alphanumeric string (mx1.domain.com). For example, any time you enter a domain in your browser, your request is forwarded through the global DNS system to the company whose NS records the domain uses and from there you will be directed to the servers of another company if you have set an IP address of the latter as an A record for your domain name. Having independent records for the website and the e-mails means that you could have your site and your emails with two different companies if you would like.