Whenever you add a domain name 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 Internet domain is added - one A record and two MX records. The first one is a numeric address, or IP address, which “tells” the domain name where its site is, while the other two are alphanumeric and they show the server that manages the emails for that particular domain name. The website and the e-mail hosting are usually regarded as one thing, while they're actually two different services. Having separate records for them will enable you to have them with different providers if you would like. As an example, some new company may 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 to the first 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 wish to open a site or send an email - either way, the provider whose name servers are used for the domain address 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'll see the needed website or your e-mail will be delivered.