Every registered domain name has a minimum of two Name Server records that show where it is hosted i.e. by using these records you point your domain name to the servers of a certain hosting company. This way, you've got both your website and your e-mails handled by the very same company. On the lower level of the Domain Name System (DNS), however, there are a number of other records, like A and MX. The former shows which server handles the website for a given domain address and is always an IP address (18.104.22.168), while the second one shows which server handles the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time you type a domain in your Internet browser, your request is forwarded through the global DNS system to the provider whose NS records the domain uses and from there you may be directed to the servers of a different company if you have set an IP address of the latter as an A record for your domain name. Having separate records for the site and the e-mails means you could have your website and your e-mails with two different providers if you'd like.