Any time you add a domain as hosted in some account, you usually set a pair of Name Servers to point it to that specific service provider. On their end, 3 records are created automatically as soon as the domain address is added - one A record and two MX records. The first one is a numeric address, or IP address, that “tells” the domain address where its website is, while the other two are alphanumeric and they reveal the server that handles the e-mails for that particular domain. The website and the e-mail hosting are often thought to be one thing, when they're in fact two different services. Having different records for them will enable you to have them with different providers if you want. For example, some new service provider may have outstanding uptime for your site, but you might not want to switch your e-mails from your current host and by using an A record to point the domain name to the former and MX records to have the emails with the latter, you could get the best of both providers. These records are checked when you wish to open a site or send an email - 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're going to see the needed site or your e-mail will be delivered.