Whenever you add a domain as hosted in some account, you typically set a pair of Name Servers to direct it to that specific service provider. On their end, three records are created automatically the moment the domain name is added - one A record and two MX records. The former is a numeric address, or IP address, which “tells” the domain where its website is, while the other two are alphanumeric and they indicate the server that deals with the emails for that particular domain. The site and the e-mail hosting are usually perceived as one thing, while they are in reality two different services. Having separate records for them will allow you to have them with different companies if you wish. For example, some new provider might 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 Internet domain to the former and MX records to have the e-mails with the latter, you will 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 Internet 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'll see the needed website or your e-mail is going to be delivered.