When you add a domain name as hosted in some account, you usually set a pair of Name Servers to point 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, that “tells” the domain where its website is, while the other two are alphanumeric and they indicate the server that manages the emails for that particular domain. The site and the email hosting are usually regarded as one thing, while they are in reality two different services. Having separate records for them will enable you to have them with different providers if you would like. For instance, some new service provider might have outstanding uptime for your site, but you may not want to switch your e-mail messages from your current host and by using an A record to point the domain name to the former and MX records to have the emails with the latter, you could get the best of both providers. These records are checked when you want to open a website or send an email - either way, 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 right web/mail server will then be contacted and you'll see the needed site or your email will be delivered.