When you add a domain as hosted in some account, you normally set a pair of Name Servers to point it to that specific company. On their end, three records are set up automatically the moment the domain name is added - one A record and two MX records. The first one is a numeric address, or IP address, which “tells” the domain address where its website is, while the other two are alphanumeric and they indicate the server that deals with the emails for that specific domain. The website and the email hosting are generally thought to be one thing, when they're actually two different services. Having different records for them will allow you to have them with different companies if you would like. For example, some new provider may have fantastic uptime for your site, but you might not want to switch your e-mails from your current host and by using an A record to point the domain name to the first and MX records to have the e-mails with the second, 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 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're going to see the needed site or your email is going to be delivered.