Whenever you add a domain as hosted in some account, you normally set a pair of Name Servers to point it to that specific company. On their end, 3 records are created automatically right after the domain name is added - one A record and two MX records. The former is a numeric address, or IP address, that “tells” the domain address where its site is, while the other two are alphanumeric and they indicate the server that handles the emails for that particular domain name. The site and the email hosting are generally considered to be one thing, while they're actually two different services. Having different records for them will enable you to have them with different companies if you would like. As an example, some new provider can have fantastic uptime for your website, but you may not want to switch your e-mails from your current host and by employing an A record to point the domain name to the first and MX records to have the e-mails with the second, you will get the best of both providers. These records are checked when you wish to open a site or send an email - either way, the provider whose name servers are used for the domain name will be contacted to retrieve the A and MX records and if you've set records different from their own, the right web/mail server will then be contacted and you'll see the needed site or your email is going to be delivered.