Every single registered domain has at least two Name Server records which show where it's hosted i.e. by using these records you point your domain name to the servers of a certain website hosting provider. That way, you have got both your site and your e-mails managed by the very same provider. On the lower level of the Domain Name System (DNS), nevertheless, there is a variety of other records, for example A and MX. The first one reveals which server manages the site for a given domain address and is always an IP address (220.127.116.11), while the second one indicates which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, whenever you enter a domain address in your Internet browser, your request is directed through the global DNS system to the provider whose NS records the domain address uses and from there you may be sent to the servers of another company in case you have set an IP address of the latter as an A record for your Internet domain. Having separate records for the site and the emails means you can have your site and your e-mails with 2 different companies if you would like.