When you add a domain name as hosted in some account, you typically set a pair of Name Servers to direct it to that specific service provider. On their end, 3 records are set up automatically when 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 address where its site is, while the other two are alphanumeric and they indicate the server that handles the e-mails for that particular domain. The website and the e-mail hosting are generally regarded as one thing, while they're in fact two different services. Having different records for them will permit you to have them with different providers if you wish. For example, some new provider may have fantastic uptime for your website, but you might not want to switch your e-mails from your current host and by using an A record to point the Internet domain to the first and MX records to have the emails with the latter, you could get the best of both providers. These records are checked when you want to open a site or send an e-mail - 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 right web/mail server will then be contacted and you are going to see the needed site or your e-mail will be delivered.