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 the moment the domain name 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 reveal the server that manages the emails for that specific domain address. The website and the e-mail hosting are generally thought to be one thing, while they are actually two different services. Having different records for them will enable you to have them with different companies if you want. For instance, some new provider can 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 address to the former and MX records to have the e-mails with the latter, you will get the best of both companies. These records are checked when you want to open a website or send an email - either way, the service provider whose name servers are used for the domain address 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 are going to see the needed website or your email will be delivered.