When you add a domain as hosted in some account, you usually set a pair of Name Servers to point it to that particular service provider. On their end, 3 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, which “tells” the domain address where its website is, while the other two are alphanumeric and they reveal the server that manages the emails for that particular domain. The website and the email hosting are typically regarded as one thing, while they're in reality two different services. Having separate records for them will allow you to have them with different companies if you'd like. For example, some new company may have fantastic uptime for your website, but you may not want to switch your emails from your current host and by using an A record to point the domain to the first and MX records to have the emails with the second, you could get the best of both providers. These records are checked when you want to open a website or send an e-mail - in either case, the provider 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're going to see the needed site or your email will be delivered.