Every single registered domain name has a minimum of 2 Name Server records which show where it's hosted i.e. by using these records you point your domain to the servers of a specific website hosting company. In this way, you've got both your site and your e-mails handled by the very same provider. On the lower level of the Domain Name System (DNS), however, there are lots of other records, for instance A and MX. The first one shows which server manages the website for a given domain and is always an IP address (188.8.131.52), while the latter indicates which server handles the emails and is always an alphanumeric string (mx1.domain.com). For example, any time you type a domain name in your Internet browser, your request is sent through the global DNS system to the provider whose NS records the domain name uses and from there you could be forwarded to the servers of a different company in case you have set an IP address of the latter as an A record for your Internet domain. Having independent records for the site and the e-mails means that you could have your site and your e-mails with 2 different providers if you would like.