Whenever you add a domain 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 when 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 indicate the server that deals with the e-mails for that particular domain address. The website and the email hosting are typically considered to be one thing, while they're in reality two different services. Having different records for them will permit you to have them with different providers if you would like. For example, some new company can have fantastic uptime for your site, but you might not want to switch your e-mails from your current host and by using an A record to point the domain name to the former and MX records to have the e-mails with the latter, you could get the best of both providers. These records are checked when you wish to open a website or send an email - either way, the service provider whose name servers are used for the domain address is going to be contacted to retrieve the A and MX records and if you've set records different from their own, the right web/mail server will then be contacted and you'll see the needed site or your email will be delivered.