Each registered domain has a minimum of 2 Name Server records that show where it is hosted i.e. by using these records you direct your domain to the servers of a certain hosting provider. That way, you've got both your site and your e-mails managed by the same company. On the lower level of the Domain Name System (DNS), however, there is a variety of other records, for instance A and MX. The first one reveals which server handles the website for a given domain and is always an IP address (123.123.123.123), while the second one indicates which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). For example, any time you type a domain name in your browser, your request is forwarded through the global DNS system to the provider whose NS records the domain name uses and from there you may be sent to the servers of a different service provider if you have set an IP address of the latter as an A record for your domain. Having different records for the website and the e-mails suggests that you can have your site and your emails with two different companies if you wish.