Whenever you add a domain name as hosted in some account, you typically set a pair of Name Servers to point it to that specific company. On their end, 3 records are created automatically right after the domain is added - one A record and two MX records. The former is a numeric address, or IP address, which “tells” the domain name where its website is, while the other two are alphanumeric and they indicate the server that manages the emails for that particular Internet domain. The website and the e-mail hosting are generally thought to be one thing, while they're actually two different services. Having different records for them will enable you to have them with different companies if you'd like. For instance, some new service provider may have outstanding 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 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 site 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've set records different from their own, the correct web/mail server will then be contacted and you will see the needed website or your email is going to be delivered.