Any time you add a domain as hosted in some account, you normally set a pair of Name Servers to direct it to that particular company. On their end, 3 records are set up automatically when the domain is added - one A record and two MX records. The former is a numeric address, or IP address, which “tells” the domain name where its website is, while the other two are alphanumeric and they indicate the server that handles the emails for that particular domain address. The website and the email hosting are usually thought to be one thing, when they are in fact two different services. Having separate records for them will enable you to have them with different providers if you want. For example, some new provider may have fantastic uptime for your site, but you might not want to switch your e-mail messages from your current host and by using an A record to point the domain to the former and MX records to have the emails with the latter, you could get the best of both companies. These records are checked whenever you want to open a site or send an e-mail - in any case, the company whose name servers are used for the domain name will 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 will see the needed site or your e-mail will be delivered.
