Every single registered domain has at least two 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 web hosting provider. That way, you've got both your site and your emails handled by the same service provider. On the lower level of the Domain Name System (DNS), on the other hand, there are lots of other records, for example A and MX. The first one shows which server handles the site for a given domain name and is always an IP address (188.8.131.52), while the latter shows which server manages the e-mails and is always an alphanumeric string (mx1.domain.com). For instance, when you enter a domain name in your browser, your request is sent through the global DNS system to the provider whose NS records the domain address uses and from there you will be directed to the servers of a different provider provided you have set an IP address of the latter as an A record for your domain name. Having independent records for the website and the e-mails means that you may have your website and your e-mails with 2 different companies if you wish.