Every registered domain has no less than 2 Name Server records which show where it is hosted i.e. by using these records you direct your domain address to the servers of a certain hosting provider. That way, you have both your website and your emails handled by the same service provider. On the lower level of the Domain Name System (DNS), nevertheless, there is a variety of other records, for instance A and MX. The first one reveals which server deals with the website for a given domain 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 illustration, whenever you type a domain name in your web browser, your request is directed through the global DNS system to the provider whose NS records the domain uses and from there you could be sent to the servers of a different provider if you have set an IP address of the latter as an A record for your domain name. Having separate records for the site and the emails means you could have your website and your e-mails with 2 different providers if you'd like.