When 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 former is a numeric address, or IP address, that “tells” the domain address where its site is, while the other two are alphanumeric and they indicate the server that deals with the e-mails for that particular domain name. The site and the e-mail hosting are typically considered to be one thing, while they are in reality two different services. Having different records for them will allow you to have them with different providers if you'd like. As an illustration, some new provider might have superb uptime for your site, 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 former and MX records to have the emails with the second, you can get the best of both providers. These records are checked when you want to open a website or send an e-mail - in any case, the provider whose name servers are used for the domain address will 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 email is going to be delivered.