Each registered domain name has at least two Name Server records that show where it's hosted i.e. by using these records you point your Internet domain to the servers of a specific website hosting company. 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), however, there are lots of other records, such as A and MX. The former shows 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 handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, any time you enter a domain name in your browser, your request is sent through the global DNS system to the provider whose NS records the domain address uses and from there you could be forwarded 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 independent records for the site and the emails means you can have your website and your emails with 2 different companies if you wish.