Any time you add a domain as hosted in some account, you typically set a pair of Name Servers to point it to that specific service provider. On their end, three records are created automatically right after the Internet domain is added - one A record and two MX records. The former is a numeric address, or IP address, which “tells” the domain address where its site is, while the other two are alphanumeric and they indicate the server that handles the e-mails for that specific domain. The site and the e-mail hosting are usually perceived as one thing, while they are actually two different services. Having independent records for them will permit you to have them with different providers if you would like. As an example, some new service provider might have superb uptime for your site, 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 address to the former and MX records to have the e-mails with the second, you can get the best of both companies. These records are checked when you want to open a site or send an email - either way, the company whose name servers are used for the domain is going to 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 will be delivered.