Every single registered domain has a minimum of two Name Server records that show where it is hosted i.e. by using these records you direct your Internet domain to the servers of a certain hosting provider. That way, you've got both your site and your emails handled by the exact same company. On the lower level of the Domain Name System (DNS), however, there are a number of other records, for example A and MX. The first one shows which server manages the site for a given Internet domain and is always an IP address (123.123.123.123), while the latter indicates which server manages the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you type a domain address in your web browser, your request is forwarded through the global DNS system to the company whose NS records the domain name uses and from there you could be forwarded to the servers of another service provider if you have set an IP address of the latter as an A record for your Internet domain. Having separate records for the website and the emails means you may have your website and your emails with 2 different providers if you would like.