Every single registered domain has a minimum of 2 Name Server records which show where it's hosted i.e. by using these records you direct your domain to the servers of a particular web hosting company. That way, you have got both your site and your emails handled by the same service provider. On the lower level of the Domain Name System (DNS), however, there is a variety of other records, like A and MX. The former reveals which server handles the site for a given domain name 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). For instance, whenever you enter a domain address in your Internet browser, your request is directed through the global DNS system to the provider whose NS records the domain address uses and from there you may be sent to the servers of another service provider in case you have set an IP address of the latter as an A record for your Internet domain. Having independent records for the website and the e-mails means that you could have your site and your emails with 2 different companies if you wish.