Every single registered domain name has a minimum of 2 Name Server records which show where it's hosted i.e. by using these records you direct your domain address to the servers of a specific hosting provider. This way, you have both your website and your e-mails managed by the very same company. On the lower level of the Domain Name System (DNS), however, there is a variety of other records, for example A and MX. The first one reveals which server handles the website for a given Internet domain and is always an IP address (184.108.40.206), while the second one reveals which server handles the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time you enter a domain address in your browser, your request is directed through the global DNS system to the company whose NS records the domain uses and from there you could be sent to the servers of another company provided you have set an IP address of the latter as an A record for your domain. Having separate records for the site and the e-mails means that you may have your site and your emails with two different companies if you would like.