Every registered domain has no less than two Name Server records which 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. This way, you've got both your site and your e-mails handled by the exact same provider. On the lower level of the Domain Name System (DNS), however, there are lots of other records, like A and MX. The first one reveals which server deals with the website for a given domain address and is always an IP address (123.123.123.123), while the second one reveals which server deals with the emails and is always an alphanumeric string (mx1.domain.com). For instance, any time you enter a domain name in your web browser, your request is forwarded through the global DNS system to the provider whose NS records the domain name uses and from there you could be forwarded to the servers of another provider provided 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 means you can have your site and your emails with two different companies if you'd like.