Every single registered domain name has no less than 2 Name Server records which show where it's hosted i.e. by using these records you direct your domain to the servers of a specific hosting provider. In this way, you have got both your website and your emails handled by the exact same provider. On the lower level of the Domain Name System (DNS), on the other hand, there are a number of other records, for instance A and MX. The first one reveals which server handles the site for a given domain and is always an IP address (123.123.123.123), while the second one indicates which server manages the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time you type a domain in your web browser, your request is forwarded through the global DNS system to the company whose NS records the domain name uses and from there you could be sent to the servers of another provider in case you have set an IP address of the latter as an A record for your domain address. Having different records for the website and the emails means you could have your site and your e-mails with 2 different providers if you'd like.