Every registered domain has a minimum of 2 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 certain web hosting company. This way, you have both your site and your e-mails managed 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 instance A and MX. The former shows which server deals with the website for a given domain address and is always an IP address (123.123.123.123), while the second one shows which server manages the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, whenever 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 will be forwarded 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 different records for the site and the emails means you can have your website and your emails with two different providers if you would like.