Any time you add a domain as hosted in some account, you usually set a pair of Name Servers to direct it to that particular company. On their end, three 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 where its site is, while the other two are alphanumeric and they reveal the server that manages the e-mails for that specific domain. The site and the email hosting are typically considered to be one thing, when they are in reality two different services. Having independent records for them will permit you to have them with different companies if you want. For instance, some new service provider can have superb uptime for your site, 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 former and MX records to have the emails with the latter, you can get the best of both companies. These records are checked when you want to open a website or send an email - either way, the provider whose name servers are used for the Internet domain 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're going to see the needed site or your e-mail is going to be delivered.
