Every single registered domain has at least two Name Server records which show where it's hosted i.e. by using these records you direct your domain name to the servers of a particular website hosting company. This way, you have got both your site and your emails managed by the exact same company. On the lower level of the Domain Name System (DNS), on the other hand, there are lots of other records, such as A and MX. The first one shows which server deals with the site for a given domain and is always an IP address (188.8.131.52), while the second one shows which server handles the emails and is always an alphanumeric string (mx1.domain.com). As an example, when you enter a domain address in your 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 forwarded to the servers of a different company provided 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 can have your website and your emails with 2 different companies if you would like.