Every registered domain has no less than 2 Name Server records that show where it's hosted i.e. by using these records you point your domain address to the servers of a specific website hosting company. That way, you've got both your website and your e-mails handled by the very same company. On the lower level of the Domain Name System (DNS), however, there is a variety of other records, for example A and MX. The former reveals which server handles the website for a given domain name and is always an IP address (188.8.131.52), while the latter reveals which server handles the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, when you enter a domain name in your web browser, your request is sent through the global DNS system to the company whose NS records the domain uses and from there you could be sent to the servers of another service provider if you have set an IP address of the latter as an A record for your Internet domain. Having independent records for the website and the e-mails means you could have your website and your emails with 2 different providers if you'd like.