Each registered domain has at least 2 Name Server records which show where it's hosted i.e. by using these records you direct your domain address to the servers of a certain website hosting provider. In this way, you have both your website and your e-mails managed by the very same company. On the lower level of the Domain Name System (DNS), nevertheless, there is a variety of other records, for instance A and MX. The former shows which server manages the website for a given domain name and is always an IP address (18.104.22.168), while the second one indicates which server manages the emails and is always an alphanumeric string (mx1.domain.com). For instance, whenever you enter a domain in your Internet browser, your request is directed through the global DNS system to the company whose NS records the domain address uses and from there you could be directed to the servers of another service provider provided 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 that you could have your site and your emails with 2 different companies if you wish.