When you add a domain 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 where its website is, while the other two are alphanumeric and they show the server that deals with the e-mails for that specific domain. The website and the e-mail hosting are often regarded as one thing, when they are actually two different services. Having independent records for them will permit you to have them with different companies if you'd like. For instance, some new provider may have exceptional uptime for your site, but you might not want to switch your emails from your current host and by employing an A record to point the Internet domain to the first and MX records to have the emails with the latter, you will get the best of both companies. These records are checked when you wish to open a website or send an email - in either case, the service provider whose name servers are used for the Internet domain 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're going to see the needed site or your email is going to be delivered.
