Each registered domain name has no less than two Name Server records that show where it's hosted i.e. by using these records you direct your Internet domain to the servers of a certain hosting company. That way, you have both your site and your e-mails managed by the very same company. On the lower level of the Domain Name System (DNS), on the other hand, there are a number of other records, for example A and MX. The former reveals which server manages the site for a given domain address and is always an IP address (18.104.22.168), while the second one reveals which server deals with the emails and is always an alphanumeric string (mx1.domain.com). For example, when you enter a domain address 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 could be directed to the servers of another service provider if you have set an IP address of the latter as an A record for your Internet domain. Having different records for the site and the e-mails means you may have your website and your emails with two different companies if you'd like.