Each 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 to the servers of a certain hosting company. In this way, you've got both your website and your e-mails handled by the exact same provider. On the lower level of the Domain Name System (DNS), however, there are a number of other records, such as A and MX. The first one shows which server deals with the site for a given Internet domain and is always an IP address (123.123.123.123), while the latter shows which server manages the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, when you type a domain name in your web browser, your request is sent through the global DNS system to the company whose NS records the domain name uses and from there you will be sent to the servers of another provider if you have set an IP address of the latter as an A record for your domain address. Having separate records for the website and the e-mails means you can have your website and your emails with 2 different companies if you'd like.