When you add a domain as hosted in some account, you normally set a pair of Name Servers to point it to that specific service provider. On their end, three records are created automatically as soon as the domain address is added - one A record and two MX records. The former is a numeric address, or IP address, which “tells” the domain address where its website is, while the other two are alphanumeric and they indicate the server that manages the emails for that specific domain address. The website and the email hosting are usually considered to be one thing, when they are actually two different services. Having separate records for them will permit you to have them with different companies if you wish. For instance, some new provider could have superb uptime for your site, 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 first and MX records to have the emails with the latter, you can get the best of both companies. These records are checked whenever you want to open a site or send an e-mail - in any case, the provider whose name servers are used for the Internet domain is going to 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'll see the needed site or your e-mail is going to be delivered.
