Every single registered domain name has at least 2 Name Server records which show where it's hosted i.e. by using these records you point your domain to the servers of a particular web hosting company. This way, you have both your site and your e-mails handled by the same provider. On the lower level of the Domain Name System (DNS), however, there are a number of other records, for instance A and MX. The former shows which server manages the site for a given Internet domain and is always an IP address (18.104.22.168), while the second one shows which server manages the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you type a domain in your Internet browser, your request is sent through the global DNS system to the company whose NS records the domain address uses and from there you will be forwarded to the servers of another service provider in case you have set an IP address of the latter as an A record for your domain name. Having different records for the site and the emails means that you could have your site and your emails with two different companies if you would like.