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, three records are set up automatically the moment the domain address is added - one A record and two MX records. The former is a numeric address, or IP address, which “tells” the domain where its site is, while the other two are alphanumeric and they show the server that deals with the emails for that particular domain address. The site and the email hosting are usually considered to be one thing, while they're actually two different services. Having independent records for them will enable you to have them with different providers if you would like. For example, some new company may have fantastic uptime for your website, but you may 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 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 - in either case, the service provider whose name servers are used for the Internet 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 will be delivered.