Every registered domain name has no less than 2 Name Server records which show where it is hosted i.e. by using these records you point your domain to the servers of a specific hosting provider. That way, you've got both your website and your emails handled by the exact same service provider. On the lower level of the Domain Name System (DNS), nevertheless, there are a number of other records, like A and MX. The first one shows which server handles the site for a given domain name and is always an IP address (123.123.123.123), while the second one reveals which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, when you enter a domain name in your browser, your request is directed through the global DNS system to the provider whose NS records the domain address uses and from there you could be forwarded to the servers of another service 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 suggests that you can have your site and your emails with 2 different companies if you would like.