When you add a domain name as hosted in some account, you normally set a pair of Name Servers to point it to that particular provider. On their end, three records are created automatically the moment the domain name is added - one A record and two MX records. The first one is a numeric address, or IP address, that “tells” the domain address where its website 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 e-mail hosting are typically regarded as one thing, while they are actually two different services. Having separate records for them will allow you to have them with different companies if you want. For example, some new company can have excellent uptime for your site, but you might not want to switch your e-mail messages from your current host and by using an A record to point the domain to the former and MX records to have the e-mails with the second, you will get the best of both providers. These records are checked when you wish to open a website or send an e-mail - in either case, the service 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'll see the needed site or your email is going to be delivered.