Each registered domain name has at least 2 Name Server records that show where it is hosted i.e. by using these records you point your Internet domain to the servers of a specific web hosting provider. This way, you have got both your website and your e-mails managed by the very same provider. On the lower level of the Domain Name System (DNS), however, there are a number of other records, like A and MX. The former 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 deals with 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 sent through the global DNS system to the provider whose NS records the domain address uses and from there you could be forwarded to the servers of another service provider provided 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 emails means that you can have your site and your emails with two different providers if you'd like.