When you add a domain name 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 when 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 name where its site is, while the other two are alphanumeric and they reveal the server that manages the emails for that specific Internet domain. The site and the email hosting are often perceived as one thing, when they're in fact two different services. Having different records for them will enable you to have them with different companies if you wish. For example, some new company may have superb uptime for your website, but you might not want to switch your emails from your current host and by using an A record to point the domain address to the first and MX records to have the e-mails with the latter, you will get the best of both providers. These records are checked when you wish to open a site or send an e-mail - either way, the provider whose name servers are used for the domain name is going to be contacted to retrieve the A and MX records and if you have 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.