Every single registered domain name has a minimum of two Name Server records which show where it is hosted i.e. by using these records you point your Internet domain to the servers of a particular web hosting provider. That 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), on the other hand, there are lots of other records, for instance A and MX. The first one shows which server deals with the site for a given Internet domain and is always an IP address (123.123.123.123), while the latter shows which server manages the e-mails and is always an alphanumeric string (mx1.domain.com). For example, whenever you enter a domain address in your web browser, your request is directed through the global DNS system to the company whose NS records the domain address uses and from there you may be directed to the servers of a different service provider provided you have set an IP address of the latter as an A record for your domain address. Having independent records for the site and the e-mails means you can have your website and your e-mails with two different providers if you would like.