Every registered domain has a minimum of 2 Name Server records which show where it is hosted i.e. by using these records you point your domain address to the servers of a certain web hosting company. In this way, you have got both your site and your e-mails handled by the same company. On the lower level of the Domain Name System (DNS), on the other hand, there are lots of other records, like A and MX. The former shows which server handles the website for a given domain address and is always an IP address (184.108.40.206), while the latter reveals which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). As an illustration, when you enter a domain name in your browser, your request is directed through the global DNS system to the company whose NS records the domain address uses and from there you will be directed to the servers of a different service provider in case you have set an IP address of the latter as an A record for your domain address. Having separate records for the website and the e-mails means that you may have your website and your e-mails with 2 different companies if you wish.