Any time you add a domain as hosted in some account, you usually set a pair of Name Servers to point it to that particular provider. On their end, 3 records are created automatically right after the domain address is added - one A record and two MX records. The former is a numeric address, or IP address, that “tells” the Internet domain where its site is, while the other two are alphanumeric and they reveal the server that deals with the emails for that particular domain address. The site and the e-mail hosting are generally perceived as one thing, while they are in reality two different services. Having different records for them will allow you to have them with different companies if you want. For example, some new company may have fantastic uptime for your website, but you might not want to switch your emails from your current host and by using an A record to point the domain address to the first and MX records to have the emails with the latter, you could get the best of both providers. These records are checked whenever you want to open a site or send an email - in any case, the service 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'll see the needed website or your email is going to be delivered.