Each registered domain has at least two Name Server records that show where it is hosted i.e. by using these records you point your domain address to the servers of a specific website hosting provider. In this way, you've got both your site and your emails managed by the very same company. On the lower level of the Domain Name System (DNS), nevertheless, there are lots of other records, for example A and MX. The former reveals which server handles the website for a given domain address and is always an IP address (123.123.123.123), while the latter reveals which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). For instance, any time you type a domain name in your browser, your request is directed through the global DNS system to the provider whose NS records the domain address uses and from there you could be forwarded to the servers of another service provider if you have set an IP address of the latter as an A record for your Internet domain. Having separate records for the site and the emails suggests that you can have your site and your e-mails with 2 different providers if you would like.