Every single registered domain name has at least two Name Server records which show where it's hosted i.e. by using these records you point your domain to the servers of a particular web hosting company. In this way, you've got both your website and your e-mails managed by the same company. On the lower level of the Domain Name System (DNS), on the other hand, there is a variety of other records, such as A and MX. The first one reveals which server manages the site for a given domain address and is always an IP address (123.123.123.123), while the second one reveals which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you enter a domain address in your 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 directed to the servers of a different provider if you have set an IP address of the latter as an A record for your domain name. Having separate records for the website and the e-mails means you can have your site and your e-mails with two different providers if you'd like.