Any time you add a domain as hosted in some account, you typically set a pair of Name Servers to direct it to that particular provider. On their end, three records are created automatically right after the domain is added - one A record and two MX records. The first one 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 emails for that particular domain name. The site and the email hosting are usually considered to be one thing, when they are actually two different services. Having different records for them will allow you to have them with different providers if you'd like. For instance, some new provider could have fantastic uptime for your website, but you might not want to switch your emails from your current host and by employing an A record to point the domain name to the former and MX records to have the emails with the second, you could get the best of both companies. These records are checked when you wish to open a website or send an email - in any case, the company whose name servers are used for the domain will be contacted to retrieve the A and MX records and if you have 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 e-mail will be delivered.