When you add a domain as hosted in some account, you typically set a pair of Name Servers to point it to that specific service provider. On their end, 3 records are set up automatically right after the domain address is added - one A record and two MX records. The first one is a numeric address, or IP address, which “tells” the domain name where its site is, while the other two are alphanumeric and they reveal the server that deals with the e-mails for that specific domain address. The website and the email hosting are often perceived as one thing, when they're actually two different services. Having different records for them will permit you to have them with different providers if you want. For instance, some new provider might have outstanding uptime for your website, but you may not want to switch your e-mail messages 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 website or send an email - in any case, the service provider whose name servers are used for the Internet domain is going to 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're going to see the needed site or your e-mail is going to be delivered.
