Every single registered domain has at least two Name Server records that show where it is hosted i.e. by using these records you point your domain to the servers of a particular web hosting company. In this way, you've got both your website and your emails managed by the same company. On the lower level of the Domain Name System (DNS), nevertheless, there are lots of other records, like A and MX. The first one shows which server deals with the site for a given domain and is always an IP address (123.123.123.123), while the latter indicates which server deals with 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 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. Having independent records for the website and the emails means that you may have your website and your e-mails with two different providers if you'd like.
