Every single registered domain name has at least two Name Server records that show where it's hosted i.e. by using these records you direct your domain address to the servers of a particular hosting provider. That way, you've got both your website and your e-mails managed by the 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 shows which server manages the site for a given Internet domain and is always an IP address (220.127.116.11), while the latter reveals which server manages the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, any time you type a domain address in your web browser, your request is directed through the global DNS system to the provider whose NS records the domain name uses and from there you may be forwarded to the servers of a different company in case you have set an IP address of the latter as an A record for your Internet domain. Having separate records for the website and the emails means you can have your website and your e-mails with 2 different companies if you'd like.