When you add a domain as hosted in some account, you usually set a pair of Name Servers to direct it to that particular service provider. On their end, three records are created automatically the moment the domain address is added - one A record and two MX records. The first one is a numeric address, or IP address, which “tells” the domain address where its site is, while the other two are alphanumeric and they reveal the server that manages the e-mails for that particular domain address. The site and the email hosting are typically perceived as one thing, while they're actually two different services. Having separate records for them will enable you to have them with different companies if you'd like. As an example, some new service provider might have exceptional uptime for your site, but you may 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 emails with the latter, you will 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 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 correct web/mail server will then be contacted and you'll see the needed site or your email is going to be delivered.