When you add a domain name as hosted in some account, you normally set a pair of Name Servers to direct it to that particular provider. On their end, three records are created automatically when the domain address is added - one A record and two MX records. The former is a numeric address, or IP address, that “tells” the domain name where its website is, while the other two are alphanumeric and they reveal the server that manages the e-mails for that particular domain. The site and the email hosting are generally perceived as one thing, while they're actually two different services. Having different records for them will enable you to have them with different providers if you wish. For example, some new provider can have outstanding uptime for your website, but you might not want to switch your emails from your current host and by employing an A record to point the domain address to the former and MX records to have the emails with the latter, 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 provider 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 website or your e-mail will be delivered.