Each registered domain name has at least two Name Server records which show where it's hosted i.e. by using these records you direct your domain name to the servers of a certain hosting company. That way, you have both your site and your emails handled by the very same service provider. On the lower level of the Domain Name System (DNS), on the other hand, there are a number of other records, such as A and MX. The first one shows which server deals with the site for a given domain name and is always an IP address (18.104.22.168), while the latter shows which server deals with the emails and is always an alphanumeric string (mx1.domain.com). For example, any time you enter a domain name in your browser, your request is directed through the global DNS system to the provider whose NS records the domain address uses and from there you may be sent to the servers of another provider if you have set an IP address of the latter as an A record for your domain. Having different records for the site and the emails means you may have your site and your e-mails with two different companies if you would like.