When you add a domain as hosted in some account, you typically 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, 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 typically regarded as one thing, while they are actually two different services. Having separate records for them will permit you to have them with different providers if you'd like. For instance, some new company may have excellent uptime for your website, but you might not want to switch your e-mails from your current host and by employing an A record to point the domain to the first and MX records to have the e-mails with the latter, you will get the best of both providers. These records are checked when you wish to open a site or send an e-mail - in either case, the provider whose name servers are used for the 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 site or your email will be delivered.