Whenever you add a domain as hosted in some account, you normally set a pair of Name Servers to point 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 first one is a numeric address, or IP address, which “tells” the domain address where its site is, while the other two are alphanumeric and they indicate the server that handles the e-mails for that specific domain. The website and the email hosting are usually considered to be one thing, when they are actually two different services. Having different records for them will allow you to have them with different providers if you'd like. For example, some new provider may have superb 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 former and MX records to have the e-mails with the second, you could 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 is going to be contacted to retrieve the A and MX records and if you have set records different from their own, the correct web/mail server will then be contacted and you're going to see the needed website or your e-mail is going to be delivered.