Every registered domain name has a minimum of two Name Server records that 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. In this way, you have both your website and your emails handled by the 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 handles the site for a given domain address and is always an IP address (123.123.123.123), while the latter shows which server handles the emails and is always an alphanumeric string (mx1.domain.com). As an example, any time you enter a domain in your Internet browser, your request is directed through the global DNS system to the provider whose NS records the domain address uses and from there you will be sent to the servers of another company if you have set an IP address of the latter as an A record for your domain address. Having separate records for the site and the emails means that you may have your website and your emails with 2 different providers if you would like.