Every registered domain name has at least 2 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 site and your e-mails managed by the very same provider. On the lower level of the Domain Name System (DNS), on the other hand, there is a variety of other records, for instance A and MX. The first one shows which server handles the site for a given Internet domain and is always an IP address (188.8.131.52), while the second one shows which server manages the emails and is always an alphanumeric string (mx1.domain.com). As an example, whenever you type a domain in your web browser, your request is sent through the global DNS system to the provider whose NS records the domain uses and from there you will be sent to the servers of another company if you have set an IP address of the latter as an A record for your domain. Having independent records for the site and the e-mails means you can have your website and your e-mails with 2 different providers if you would like.