Every single registered domain name has a minimum of two Name Server records that show where it is hosted i.e. by using these records you direct your Internet domain to the servers of a certain hosting company. This way, you have both your site and your emails managed by the same company. 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 shows which server handles the site for a given domain address and is always an IP address (18.104.22.168), while the second one shows which server deals with the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, when you type a domain address 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 may be sent to the servers of another provider if you have set an IP address of the latter as an A record for your domain address. Having independent records for the website and the e-mails means that you may have your website and your e-mails with two different companies if you wish.