Any time you add a domain name as hosted in some account, you usually set a pair of Name Servers to direct it to that particular provider. On their end, three records are set up automatically as soon as the domain address is added - one A record and two MX records. The first one is a numeric address, or IP address, which “tells” the domain where its website is, while the other two are alphanumeric and they reveal the server that manages the emails for that particular domain address. The site and the email hosting are usually thought to be one thing, while they're actually two different services. Having separate records for them will allow you to have them with different companies if you wish. As an example, some new provider may 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 domain to the first and MX records to have the e-mails with the latter, you will get the best of both companies. These records are checked when you wish to open a website or send an email - either way, the company whose name servers are used for the domain name 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 will see the needed site or your e-mail is going to be delivered.