Each registered domain has no less than 2 Name Server records that show where it's hosted i.e. by using these records you point your domain name to the servers of a certain web hosting provider. In this way, you've got both your site and your emails handled by the same provider. On the lower level of the Domain Name System (DNS), nevertheless, there are lots of other records, such as A and MX. The former shows which server deals with the site for a given domain address and is always an IP address (123.123.123.123), while the latter indicates which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). For instance, whenever you type a domain in your Internet browser, your request is sent through the global DNS system to the provider whose NS records the domain uses and from there you will 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 separate records for the site and the emails means that you could have your site and your e-mails with two different companies if you'd like.