When you add a domain name as hosted in some account, you typically set a pair of Name Servers to direct it to that particular company. On their end, three 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 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 name. The website and the e-mail hosting are often considered to be one thing, while they're in fact two different services. Having different records for them will enable you to have them with different companies if you would like. As an example, some new provider could have superb 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 former and MX records to have the e-mails with the second, you could get the best of both companies. These records are checked when you want to open a website or send an email - in either case, 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 have set records different from their own, the right web/mail server will then be contacted and you're going to see the needed site or your e-mail will be delivered.