Any time you add a domain name as hosted in some account, you normally set a pair of Name Servers to point it to that specific service provider. On their end, 3 records are set up automatically as soon as the Internet domain is added - one A record and two MX records. The former is a numeric address, or IP address, which “tells” the domain address where its site is, while the other two are alphanumeric and they show the server that deals with the e-mails for that specific Internet domain. The site and the e-mail hosting are usually perceived as one thing, while they're in fact two different services. Having separate records for them will permit you to have them with different companies if you want. For example, some new company could have fantastic uptime for your website, but you may 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 e-mail - either way, the company whose name servers are used for the domain is going to be contacted to retrieve the A and MX records and if you've set records different from their own, the correct web/mail server will then be contacted and you're going to see the needed site or your e-mail will be delivered.