Any time you add a domain as hosted in some account, you usually set a pair of Name Servers to point it to that specific service provider. On their end, three records are created 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 name where its website is, while the other two are alphanumeric and they reveal the server that handles the emails for that particular domain address. The site and the email hosting are usually considered to be one thing, when they are in reality two different services. Having independent records for them will allow you to have them with different companies if you want. For example, some new provider might have excellent uptime for your site, but you may not want to switch your e-mail messages from your current host and by employing an A record to point the domain name to the former and MX records to have the emails with the latter, you could get the best of both companies. These records are checked whenever you want to open a website or send an email - in either case, the company whose name servers are used for the domain is going to 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 are going to see the needed website or your e-mail is going to be delivered.