Every registered domain name has no less than 2 Name Server records which show where it's hosted i.e. by using these records you direct your domain name to the servers of a certain hosting provider. That way, you have both your website and your emails managed by the 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 website for a given domain address 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). As an illustration, when you type a domain name in your web browser, your request is directed through the global DNS system to the company whose NS records the domain uses and from there you will be directed to the servers of another provider in case you have set an IP address of the latter as an A record for your domain address. Having separate records for the website and the e-mails suggests that you may have your website and your emails with 2 different companies if you would like.