When you add a domain as hosted in some account, you typically set a pair of Name Servers to direct it to that particular provider. On their end, three 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, which “tells” the Internet domain where its site is, while the other two are alphanumeric and they show the server that manages the e-mails for that particular domain. The site and the email hosting are often regarded as one thing, when they are in fact two different services. Having different records for them will allow you to have them with different companies if you'd like. For instance, some new provider could have excellent uptime for your site, but you may not want to switch your e-mails from your current host and by employing an A record to point the domain address to the former 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 want to open a site or send an e-mail - in any case, the provider whose name servers are used for the domain name will be contacted to retrieve the A and MX records and if you have set records different from their own, the right web/mail server will then be contacted and you will see the needed site or your e-mail is going to be delivered.