Every registered domain has a minimum of two Name Server records which show where it's hosted i.e. by using these records you direct your domain name to the servers of a particular web hosting company. That way, you've got both your website and your e-mails managed by the exact same company. On the lower level of the Domain Name System (DNS), nevertheless, there are a number of other records, for example A and MX. The former reveals which server manages the website for a given domain address and is always an IP address (188.8.131.52), while the latter reveals which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, any time you type a domain address in your web browser, your request is forwarded through the global DNS system to the provider whose NS records the domain address uses and from there you may be forwarded to the servers of another company provided you have set an IP address of the latter as an A record for your domain name. Having independent records for the site and the emails means that you could have your site and your e-mails with two different companies if you wish.