Each 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 specific web hosting company. That way, you have both your site and your e-mails managed by the same company. On the lower level of the Domain Name System (DNS), nevertheless, there is a variety of other records, like A and MX. The first one shows which server deals with the website for a given Internet domain and is always an IP address (123.123.123.123), while the second one reveals which server handles the emails and is always an alphanumeric string (mx1.domain.com). For example, when you enter a domain in your Internet browser, your request is forwarded through the global DNS system to the company whose NS records the domain uses and from there you will be directed to the servers of a different service provider provided you have set an IP address of the latter as an A record for your domain. Having different records for the site and the emails means you could have your site and your e-mails with 2 different providers if you would like.