Each registered domain has a minimum of 2 Name Server records which show where it's hosted i.e. by using these records you point your domain address to the servers of a certain hosting company. This way, you have both your site and your emails managed by the exact same service provider. On the lower level of the Domain Name System (DNS), nevertheless, there are lots of other records, such as A and MX. The first one reveals which server manages the site for a given Internet domain and is always an IP address (188.8.131.52), while the latter shows which server deals with the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, whenever you type a domain in your browser, your request is sent through the global DNS system to the company whose NS records the domain address uses and from there you may be forwarded to the servers of a different company if you have set an IP address of the latter as an A record for your domain. Having different records for the site and the emails means you may have your website and your emails with 2 different companies if you would like.