Every registered domain has at least two Name Server records that show where it's hosted i.e. by using these records you direct your domain name to the servers of a specific web hosting provider. In this way, you have both your site and your emails managed by the very same provider. On the lower level of the Domain Name System (DNS), nevertheless, there are a number of other records, for instance A and MX. The first one shows which server deals with the website for a given domain and is always an IP address (123.123.123.123), while the second one reveals which server manages the emails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you enter a domain 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 will be directed 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 independent records for the website and the e-mails means that you could have your site and your e-mails with two different companies if you would like.