Any time you add a domain as hosted in some account, you usually set a pair of Name Servers to direct it to that specific provider. On their end, 3 records are set up automatically right after 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 address where its site is, while the other two are alphanumeric and they show the server that handles the e-mails for that particular domain. The site and the e-mail hosting are generally thought to be 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 wish. For example, some new company can have superb uptime for your site, but you may not want to switch your e-mails from your current host and by using an A record to point the domain address to the former and MX records to have the e-mails with the latter, you will get the best of both providers. These records are checked when you wish to open a website or send an e-mail - in either case, the provider 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 right web/mail server will then be contacted and you're going to see the needed site or your email is going to be delivered.