Whenever you add a domain as hosted in some account, you usually set a pair of Name Servers to direct it to that particular provider. On their end, 3 records are set up automatically as soon as the domain is added - one A record and two MX records. The first one is a numeric address, or IP address, which “tells” the domain where its website is, while the other two are alphanumeric and they indicate the server that deals with the emails for that specific domain address. The website and the e-mail hosting are typically regarded as one thing, while they are in fact two different services. Having different records for them will allow you to have them with different providers if you wish. For example, some new company could have fantastic uptime for your site, but you might not want to switch your e-mail messages from your current host and by using an A record to point the Internet domain to the first and MX records to have the emails with the second, you will get the best of both providers. These records are checked when you wish to open a site or send an email - in either case, the company whose name servers are used for the domain address is going to be contacted to retrieve the A and MX records and if you've set records different from their own, the right web/mail server will then be contacted and you're going to see the needed website or your e-mail is going to be delivered.