Every single registered domain has a minimum of 2 Name Server records that show where it's hosted i.e. by using these records you point your domain to the servers of a particular web hosting company. In this way, you have got both your site and your emails managed by the exact same provider. On the lower level of the Domain Name System (DNS), nevertheless, there are lots of other records, for example A and MX. The former reveals which server manages the website for a given Internet domain and is always an IP address (123.123.123.123), while the second one shows which server deals with the emails and is always an alphanumeric string (mx1.domain.com). As an example, when you type a domain name in your browser, your request is sent through the global DNS system to the company whose NS records the domain address uses and from there you may be forwarded to the servers of a different provider in case you have set an IP address of the latter as an A record for your domain address. Having independent records for the site and the emails means that you may have your website and your e-mails with two different companies if you would like.