When you add a domain as hosted in some account, you normally set a pair of Name Servers to direct it to that particular service provider. On their end, 3 records are created automatically as soon as the Internet domain is added - one A record and two MX records. The first one is a numeric address, or IP address, that “tells” the domain name where its site is, while the other two are alphanumeric and they reveal the server that deals with the emails for that particular domain name. The site and the email hosting are typically thought to be one thing, while they're in fact two different services. Having independent records for them will allow you to have them with different companies if you wish. As an example, some new company can have outstanding uptime for your website, but you might not want to switch your e-mail messages from your current host and by employing an A record to point the Internet domain to the former and MX records to have the e-mails with the second, you can get the best of both companies. These records are checked whenever you want to open a site or send an e-mail - either way, the service provider whose name servers are used for the Internet domain is going to be contacted to retrieve the A and MX records and if you've set records different from their own, the correct web/mail server will then be contacted and you'll see the needed website or your e-mail will be delivered.