Every registered domain name has at least 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 certain hosting provider. This way, you have both your site and your emails handled by the 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 deals with the site for a given domain name and is always an IP address (123.123.123.123), while the second one indicates which server deals with the e-mails and is always an alphanumeric string (mx1.domain.com). As an example, any time you enter a domain in your browser, your request is directed through the global DNS system to the company whose NS records the domain address uses and from there you could be directed to the servers of another company if you have set an IP address of the latter as an A record for your domain name. Having different records for the site and the emails means that you can have your site and your e-mails with two different providers if you would like.