Every single registered domain name has at least 2 Name Server records that show where it's hosted i.e. by using these records you direct your domain name to the servers of a particular web hosting company. That way, you have both your website and your e-mails managed by the very same service provider. On the lower level of the Domain Name System (DNS), on the other hand, there is a variety of other records, such as A and MX. The first one reveals which server handles the website 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 instance, whenever you type a domain address in your browser, your request is sent 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 another company in case you have set an IP address of the latter as an A record for your domain. Having independent records for the site and the e-mails means you could have your website and your e-mails with 2 different companies if you wish.