Each registered domain has a minimum of 2 Name Server records which show where it is hosted i.e. by using these records you point your domain to the servers of a specific web hosting provider. This way, you've got both your website and your e-mails handled by the exact same service provider. On the lower level of the Domain Name System (DNS), however, there are a number of other records, for example A and MX. The former shows which server manages the site for a given Internet domain and is always an IP address (18.104.22.168), while the second one shows which server handles the emails and is always an alphanumeric string (mx1.domain.com). For instance, when you enter a domain address in your Internet browser, your request is directed through the global DNS system to the provider whose NS records the domain uses and from there you could be directed to the servers of a different company 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 emails means you could have your site and your emails with 2 different providers if you'd like.