Every single registered domain has no less than 2 Name Server records which show where it's hosted i.e. by using these records you point your domain name to the servers of a certain website hosting provider. In this way, you've got both your site and your emails managed by the exact same company. 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 former shows which server deals with the website for a given domain name and is always an IP address (18.104.22.168), while the latter indicates which server deals with the emails and is always an alphanumeric string (mx1.domain.com). For example, when you type a domain in your web 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 a different provider provided you have set an IP address of the latter as an A record for your domain. Having independent records for the website and the emails suggests that you may have your site and your emails with two different companies if you wish.