Whenever you add a domain as hosted in some account, you usually set a pair of Name Servers to point it to that specific provider. On their end, 3 records are created automatically when the domain is added - one A record and two MX records. The former is a numeric address, or IP address, that “tells” the Internet domain where its website is, while the other two are alphanumeric and they indicate the server that handles the e-mails for that particular domain name. The site and the email hosting are often regarded as one thing, when they're in reality two different services. Having independent records for them will permit you to have them with different providers if you would like. For example, some new provider might 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 Internet domain to the first and MX records to have the e-mails with the latter, you can get the best of both providers. These records are checked when you wish to open a website or send an email - either way, the company whose name servers are used for the domain name will 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 will see the needed site or your email will be delivered.