Any time you add a domain as hosted in some account, you usually set a pair of Name Servers to point it to that specific service provider. On their end, 3 records are set up automatically as soon as the domain name is added - one A record and two MX records. The former is a numeric address, or IP address, that “tells” the Internet domain where its website is, while the other two are alphanumeric and they show the server that handles the e-mails for that particular domain address. The site and the e-mail hosting are generally considered to be one thing, while they're actually two different services. Having separate records for them will allow you to have them with different companies if you want. As an example, some new service provider may have outstanding uptime for your website, but you may not want to switch your emails from your current host and by using an A record to point the domain to the first and MX records to have the e-mails with the latter, you could get the best of both companies. These records are checked when you wish to open a site or send an e-mail - in any case, 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've set records different from their own, the correct web/mail server will then be contacted and you're going to see the needed site or your email is going to be delivered.