Each registered domain has no less than two Name Server records that show where it is hosted i.e. by using these records you point your domain name to the servers of a certain website hosting company. That way, you've got both your website and your emails managed by the same service provider. 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 reveals which server manages the website for a given Internet domain and is always an IP address (123.123.123.123), while the second one shows which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, when you type a domain name in your browser, your request is sent through the global DNS system to the provider whose NS records the domain uses and from there you may be sent to the servers of another company 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 emails suggests that you may have your site and your e-mails with 2 different providers if you wish.