Every registered domain has at least 2 Name Server records that show where it's hosted i.e. by using these records you direct your domain name to the servers of a particular web hosting provider. In this way, you have both your website and your e-mails handled by the exact same service provider. On the lower level of the Domain Name System (DNS), however, there is a variety of other records, such as A and MX. The former reveals which server handles the site for a given domain and is always an IP address (188.8.131.52), while the second one indicates which server deals with the emails and is always an alphanumeric string (mx1.domain.com). As an example, whenever you type a domain name in your web browser, your request is forwarded through the global DNS system to the company whose NS records the domain uses and from there you may be forwarded to the servers of a different service provider in case you have set an IP address of the latter as an A record for your domain. Having independent records for the website and the emails suggests that you could have your site and your emails with two different providers if you wish.