Whenever you add a domain as hosted in some account, you normally set a pair of Name Servers to direct it to that specific service provider. On their end, three records are set up automatically the moment the domain address is added - one A record and two MX records. The former is a numeric address, or IP address, that “tells” the domain address where its site is, while the other two are alphanumeric and they show the server that manages the emails for that specific domain address. The site and the email hosting are generally perceived as one thing, while they're in reality two different services. Having independent records for them will allow you to have them with different companies if you would like. As an illustration, some new provider can have exceptional 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 Internet domain to the first and MX records to have the emails with the latter, you could get the best of both companies. These records are checked when you want 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 right web/mail server will then be contacted and you will see the needed site or your email will be delivered.
