Every single registered domain has a minimum of 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 specific website hosting provider. That way, you have got both your site and your e-mails handled by the very same company. On the lower level of the Domain Name System (DNS), on the other hand, there are a number of other records, for instance A and MX. The former reveals which server handles the site for a given Internet domain and is always an IP address (18.104.22.168), while the second one reveals which server deals with the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you enter a domain in your web browser, your request is forwarded through the global DNS system to the company whose NS records the domain address uses and from there you will be directed to the servers of another provider in case you have set an IP address of the latter as an A record for your domain. Having independent records for the website and the e-mails means that you can have your site and your emails with two different companies if you would like.