Every single registered domain name has a minimum of 2 Name Server records that show where it is hosted i.e. by using these records you point your domain to the servers of a certain web hosting provider. In this way, you've got both your website and your emails managed by the same provider. On the lower level of the Domain Name System (DNS), on the other hand, there are a number of other records, such as A and MX. The former shows which server deals with the website for a given domain name and is always an IP address (220.127.116.11), while the second one shows which server handles the e-mails and is always an alphanumeric string (mx1.domain.com). For example, any time you enter 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 may be directed to the servers of another company if you have set an IP address of the latter as an A record for your domain. Having different records for the website and the emails means you can have your site and your emails with two different providers if you'd like.