When 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 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, that “tells” the Internet domain where its website is, while the other two are alphanumeric and they show the server that deals with the emails for that specific Internet domain. The site and the e-mail hosting are generally considered to be one thing, while they are in fact 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 provider may have fantastic uptime for your site, but you may not want to switch your emails from your current host and by using 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 companies. These records are checked when you want to open a site or send an e-mail - in any case, the company 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 correct web/mail server will then be contacted and you will see the needed website or your email is going to be delivered.
