When you add a domain name as hosted in some account, you typically set a pair of Name Servers to direct it to that particular company. On their end, three records are set up 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 handles the e-mails for that specific domain. The site and the e-mail hosting are often perceived as one thing, when they're in fact two different services. Having independent records for them will allow you to have them with different companies if you wish. For example, some new provider could have superb uptime for your site, but you might not want to switch your e-mail messages from your current host and by employing an A record to point the Internet domain to the former and MX records to have the emails with the latter, you can get the best of both providers. These records are checked whenever you want to open a site or send an email - in any case, the provider whose name servers are used for the domain address will be contacted to retrieve the A and MX records and if you've set records different from their own, the correct 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.
