Any time you add a domain as hosted in some account, you normally set a pair of Name Servers to point it to that particular company. On their end, three records are set up 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, which “tells” the domain name where its site is, while the other two are alphanumeric and they reveal the server that handles the emails for that specific domain. The site and the e-mail hosting are generally considered to be one thing, when they're in fact two different services. Having independent records for them will permit you to have them with different providers if you wish. For example, some new service provider might have fantastic uptime for your website, but you might not want to switch your emails from your current host and by employing an A record to point the domain to the first and MX records to have the emails with the latter, you can get the best of both providers. These records are checked whenever you want to open a website or send an e-mail - in any case, the provider whose name servers are used for the domain is going to be contacted to retrieve the A and MX records and if you have set records different from their own, the correct web/mail server will then be contacted and you'll see the needed website or your email is going to be delivered.