Every single registered domain name has no less than 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 company. That way, you've got 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, such as A and MX. The first one reveals which server deals with the site for a given domain address and is always an IP address (123.123.123.123), while the second one shows which server manages the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you enter a domain address in your browser, your request is directed through the global DNS system to the provider whose NS records the domain uses and from there you could be directed to the servers of another company provided you have set an IP address of the latter as an A record for your domain. Having different records for the website and the e-mails means you could have your site and your e-mails with two different providers if you would like.