Each registered domain name has a minimum of two Name Server records which show where it's hosted i.e. by using these records you direct your domain to the servers of a certain website hosting company. This way, you've got both your website and your emails handled by the exact same company. On the lower level of the Domain Name System (DNS), however, there are lots of other records, for example A and MX. The first one reveals which server manages the site for a given domain name and is always an IP address (123.123.123.123), while the latter indicates which server manages the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time you enter a domain in your Internet browser, your request is sent through the global DNS system to the provider whose NS records the domain uses and from there you will be sent to the servers of a different provider in case you have set an IP address of the latter as an A record for your domain name. Having different records for the website and the emails means that you can have your site and your e-mails with 2 different providers if you wish.