Every single registered domain name has no less than two Name Server records which show where it is hosted i.e. by using these records you direct your domain to the servers of a particular web hosting company. This way, you have both your site and your emails managed by the same provider. On the lower level of the Domain Name System (DNS), however, there are a number of other records, for instance A and MX. The first one shows which server handles the website for a given domain name and is always an IP address (123.123.123.123), while the second one indicates which server handles the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you enter a domain in your browser, your request is directed through the global DNS system to the company whose NS records the domain name uses and from there you could be forwarded to the servers of a different service provider in case you have set an IP address of the latter as an A record for your domain. Having independent records for the site and the emails means you may have your site and your e-mails with 2 different providers if you'd like.