Any time you add a domain name as hosted in some account, you normally set a pair of Name Servers to point it to that specific service provider. On their end, 3 records are created automatically when the Internet domain is added - one A record and two MX records. The first one is a numeric address, or IP address, that “tells” the domain address where its website is, while the other two are alphanumeric and they indicate the server that handles the e-mails for that specific domain name. The website and the e-mail hosting are often perceived as one thing, while they are in fact two different services. Having independent records for them will permit you to have them with different providers if you'd like. For example, some new company might have fantastic uptime for your website, but you might not want to switch your e-mail messages from your current host and by employing an A record to point the domain address to the first and MX records to have the e-mails with the latter, you can get the best of both companies. These records are checked when you want to open a site or send an e-mail - in either case, the company whose name servers are used for the domain 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 email is going to be delivered.
