When 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 set up automatically as soon as 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 show the server that handles the e-mails for that particular domain. The website and the e-mail hosting are often thought to be one thing, while they're in fact two different services. Having different records for them will permit you to have them with different companies if you wish. For example, some new company might have fantastic uptime for your website, but you may not want to switch your e-mails from your current host and by using an A record to point the domain name to the first and MX records to have the emails with the latter, you will get the best of both providers. These records are checked when you want to open a site or send an email - in any case, the service provider whose name servers are used for the domain name is going to 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 will see the needed site or your email will be delivered.