When you add a domain name as hosted in some account, you usually set a pair of Name Servers to point it to that particular provider. On their end, three records are created automatically as soon as the domain 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 show the server that manages the emails for that particular domain name. The website and the e-mail hosting are generally considered to be one thing, when they are actually two different services. Having independent records for them will permit you to have them with different companies if you want. As an example, some new company may have outstanding uptime for your site, but you may 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 could get the best of both companies. These records are checked when you wish to open a website or send an e-mail - either way, the 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 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 email will be delivered.