Whenever you add a domain as hosted in some account, you typically set a pair of Name Servers to point it to that specific provider. On their end, three records are created automatically the moment the Internet domain is added - one A record and two MX records. The former is a numeric address, or IP address, that “tells” the domain where its site is, while the other two are alphanumeric and they indicate the server that handles the e-mails for that particular Internet domain. The site and the email hosting are generally perceived as one thing, while they're in fact two different services. Having separate records for them will allow you to have them with different providers if you would like. For example, some new service provider may have superb uptime for your website, but you might not want to switch your e-mails from your current host and by using an A record to point the domain to the former 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 website or send an email - either way, the provider whose name servers are used for the domain name 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'll see the needed website or your email will be delivered.