Whenever you add a domain name as hosted in some account, you typically set a pair of Name Servers to point it to that specific company. On their end, three records are created automatically the moment the domain name 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 show the server that manages the e-mails for that specific domain. The site and the email hosting are typically considered to be one thing, when they're in fact two different services. Having separate records for them will permit you to have them with different providers if you'd like. For example, some new service provider could 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 address to the former and MX records to have the emails with the second, you could get the best of both companies. These records are checked when you wish to open a website or send an e-mail - either way, 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 are going to see the needed website or your e-mail will be delivered.