When you add a domain as hosted in some account, you usually set a pair of Name Servers to direct it to that particular company. On their end, three records are set up automatically the moment the domain is added - one A record and two MX records. The first one is a numeric address, or IP address, which “tells” the Internet domain where its website is, while the other two are alphanumeric and they reveal the server that deals with the emails for that particular Internet domain. The website and the e-mail hosting are generally considered to be one thing, when they're in reality two different services. Having independent records for them will allow you to have them with different providers if you would like. For example, some new company can have fantastic uptime for your site, but you might not want to switch your emails from your current host and by using an A record to point the domain to the former and MX records to have the e-mails with the latter, you can get the best of both companies. These records are checked whenever you want to open a website or send an email - in any case, the company whose name servers are used for the domain address 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 are going to see the needed site or your email is going to be delivered.