Any time you add a domain as hosted in some account, you usually set a pair of Name Servers to direct it to that specific company. On their end, 3 records are set up automatically right after 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 where its website is, while the other two are alphanumeric and they reveal the server that manages the emails for that particular domain. The site and the email hosting are typically perceived as one thing, while they're in fact two different services. Having separate records for them will permit you to have them with different companies if you wish. For example, some new company may have excellent uptime for your site, but you may not want to switch your emails from your current host and by employing an A record to point the domain name to the former and MX records to have the emails with the second, you could get the best of both providers. These records are checked when you want to open a site or send an email - in any case, the provider whose name servers are used for the Internet domain will 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 are going to see the needed website or your e-mail is going to be delivered.