Each registered domain has at least two Name Server records which show where it is hosted i.e. by using these records you point your domain to the servers of a specific hosting provider. That way, you've got both your site and your e-mails managed by the same service provider. On the lower level of the Domain Name System (DNS), however, there is a variety of other records, like A and MX. The former shows which server handles the website for a given domain name and is always an IP address (188.8.131.52), while the latter indicates which server deals with the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, 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 directed to the servers of another company 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 emails suggests that you may have your site and your e-mails with two different providers if you wish.