Each registered domain name has at least two Name Server records that show where it is hosted i.e. by using these records you direct your domain address to the servers of a certain hosting provider. In this way, you have both your website and your e-mails handled by the exact same provider. On the lower level of the Domain Name System (DNS), however, there are a number of other records, for example A and MX. The former shows which server handles the site for a given domain and is always an IP address (188.8.131.52), while the second one indicates which server deals with the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, when you enter a domain address in your browser, your request is sent through the global DNS system to the provider whose NS records the domain uses and from there you will be directed to the servers of another service provider provided you have set an IP address of the latter as an A record for your domain. Having independent records for the website and the e-mails means you may have your website and your e-mails with 2 different companies if you would like.