Each registered domain has no less than two Name Server records which show where it's hosted i.e. by using these records you direct your domain to the servers of a certain web hosting provider. In this way, you've got both your site and your emails handled by the very same company. On the lower level of the Domain Name System (DNS), however, there are lots of other records, like A and MX. The former reveals which server manages the website for a given domain name and is always an IP address (188.8.131.52), while the latter shows which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you type a domain address 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 may be sent to the servers of a different provider if you have set an IP address of the latter as an A record for your domain name. Having independent records for the site and the e-mails means that you may have your site and your e-mails with 2 different providers if you'd like.