Every single registered domain has at least 2 Name Server records which show where it's hosted i.e. by using these records you point your Internet domain to the servers of a certain website hosting company. This 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), however, there is a variety of other records, like A and MX. The former shows which server deals with the site for a given domain and is always an IP address (184.108.40.206), while the latter shows which server manages the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time you type a domain name in your web browser, your request is directed through the global DNS system to the company whose NS records the domain address uses and from there you could be sent to the servers of a different provider in case you have set an IP address of the latter as an A record for your Internet domain. Having independent records for the site and the e-mails means that you may have your site and your e-mails with two different providers if you would like.