When you add a domain name as hosted in some account, you normally set a pair of Name Servers to direct it to that specific provider. On their end, three records are created automatically the moment the 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 show the server that manages the e-mails for that specific Internet domain. The website and the email hosting are usually considered to be one thing, while they're actually two different services. Having different records for them will allow you to have them with different companies if you would like. For instance, some new company may have superb uptime for your website, but you might not want to switch your e-mail messages from your current host and by using an A record to point the domain to the former and MX records to have the emails with the second, you can get the best of both companies. These records are checked whenever you want to open a site or send an e-mail - in either case, the service provider whose name servers are used for the Internet domain is going to be contacted to retrieve the A and MX records and if you've 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 email is going to be delivered.