Each registered domain has a minimum of two Name Server records which show where it's hosted i.e. by using these records you point your domain name to the servers of a specific web hosting company. That way, you've got both your website and your emails handled by the very same provider. On the lower level of the Domain Name System (DNS), nevertheless, there is a variety of other records, like A and MX. The first one reveals which server handles the site for a given Internet domain and is always an IP address (18.104.22.168), while the second one indicates which server deals with the emails and is always an alphanumeric string (mx1.domain.com). For instance, any time 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 name uses and from there you will be forwarded 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 separate records for the site and the emails means you may have your website and your emails with two different companies if you would like.