Each registered domain name has at least 2 Name Server records which show where it's hosted i.e. by using these records you direct your Internet domain to the servers of a specific hosting company. This way, you have got both your site and your e-mails handled by the same provider. On the lower level of the Domain Name System (DNS), on the other hand, there are a number of other records, like A and MX. The former shows which server handles the website for a given Internet domain and is always an IP address (188.8.131.52), while the second one reveals which server deals with the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time 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 name uses and from there you could be sent to the servers of a different company provided you have set an IP address of the latter as an A record for your domain name. Having independent records for the site and the e-mails means you may have your site and your e-mails with 2 different companies if you'd like.