When you add a domain name as hosted in some account, you typically set a pair of Name Servers to point it to that particular company. On their end, 3 records are created automatically the moment the domain address is added - one A record and two MX records. The former is a numeric address, or IP address, that “tells” the domain name where its site is, while the other two are alphanumeric and they show the server that manages the e-mails for that particular domain. The website and the e-mail hosting are typically regarded as one thing, while they're in reality two different services. Having independent records for them will allow you to have them with different companies if you'd like. For instance, some new provider can have excellent uptime for your website, but you might not want to switch your e-mails 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 second, you could get the best of both providers. These records are checked when you wish to open a site or send an e-mail - either way, the company whose name servers are used for the domain address is going to be contacted to retrieve the A and MX records and if you have set records different from their own, the right web/mail server will then be contacted and you'll see the needed site or your e-mail is going to be delivered.