Whenever you add a domain as hosted in some account, you typically set a pair of Name Servers to point it to that particular provider. On their end, three records are created automatically right after the domain is added - one A record and two MX records. The first one is a numeric address, or IP address, which “tells” the domain where its website is, while the other two are alphanumeric and they indicate the server that manages the e-mails for that particular domain. The site and the e-mail hosting are usually perceived as one thing, when they're actually two different services. Having different records for them will enable you to have them with different providers if you'd like. For example, some new service provider may have fantastic uptime for your website, but you might not want to switch your e-mail messages from your current host and by employing an A record to point the domain to the former and MX records to have the emails with the latter, you could get the best of both companies. These records are checked when you want to open a site or send an email - either way, the 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 are going to see the needed site or your e-mail is going to be delivered.