When you add a domain as hosted in some account, you usually set a pair of Name Servers to point it to that particular company. On their end, 3 records are created automatically right after the Internet domain is added - one A record and two MX records. The former is a numeric address, or IP address, that “tells” the domain where its site is, while the other two are alphanumeric and they indicate the server that manages the e-mails for that particular domain name. The website and the e-mail hosting are typically perceived as one thing, when they're actually two different services. Having independent records for them will permit you to have them with different providers if you'd like. As an example, some new company may have superb uptime for your site, but you may not want to switch your emails from your current host and by employing an A record to point the domain to the former and MX records to have the e-mails with the latter, you could get the best of both companies. These records are checked when you want to open a website or send an e-mail - in any case, the company 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 right web/mail server will then be contacted and you're going to see the needed website or your e-mail is going to be delivered.
