Any time you add a domain name as hosted in some account, you usually set a pair of Name Servers to point it to that specific provider. On their end, three records are created automatically the moment the domain address is added - one A record and two MX records. The former is a numeric address, or IP address, which “tells” the Internet domain where its website is, while the other two are alphanumeric and they indicate the server that deals with the e-mails for that particular domain name. The site and the e-mail hosting are usually thought to be one thing, while they're in reality two different services. Having different records for them will enable you to have them with different providers if you wish. For instance, some new service provider can have outstanding uptime for your site, but you might not want to switch your emails from your current host and by using an A record to point the domain name to the first and MX records to have the emails with the latter, you will get the best of both providers. These records are checked whenever you want to open a website or send an e-mail - in any case, the service provider whose name servers are used for the 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 will see the needed site or your e-mail will be delivered.