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 to the servers of a certain web hosting provider. That way, you have both your site and your emails managed by the same provider. On the lower level of the Domain Name System (DNS), nevertheless, there are lots of other records, like A and MX. The first one shows which server handles the site for a given domain address and is always an IP address (123.123.123.123), while the second one shows which server manages the emails and is always an alphanumeric string (mx1.domain.com). For instance, whenever you type a domain name in your browser, your request is sent through the global DNS system to the company whose NS records the domain name uses and from there you could 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 e-mails suggests that you could have your site and your e-mails with two different providers if you wish.