Each 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 to the servers of a certain web hosting provider. That way, you've got both your website and your e-mails managed by the exact same 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 former 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 handles the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time you type a domain name in your web 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 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 emails suggests that you can have your website and your emails with 2 different companies if you wish.