Every single registered domain has a minimum of two Name Server records that show where it is hosted i.e. by using these records you point your domain name to the servers of a specific hosting company. That way, you have both your website and your e-mails handled by the very same service provider. On the lower level of the Domain Name System (DNS), on the other hand, there are lots of other records, like A and MX. The former shows which server handles the site for a given Internet domain and is always an IP address (188.8.131.52), while the second one reveals which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time you type a domain name in your Internet browser, your request is forwarded through the global DNS system to the provider whose NS records the domain uses and from there you may be directed to the servers of another company if 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 can have your website and your emails with two different providers if you'd like.