Every registered domain has no less than 2 Name Server records which show where it's hosted i.e. by using these records you direct your domain to the servers of a particular web hosting company. In this way, you've got both your site and your emails handled by the exact same service provider. On the lower level of the Domain Name System (DNS), however, there are a number of other records, like A and MX. The first one shows which server handles the site for a given Internet domain and is always an IP address (123.123.123.123), while the second one indicates which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). For instance, whenever you type a domain address in your web browser, your request is directed through the global DNS system to the provider whose NS records the domain address uses and from there you could be directed to the servers of a different provider provided you have set an IP address of the latter as an A record for your Internet domain. Having separate records for the website and the emails suggests that you can have your website and your emails with two different providers if you wish.