Any time you add a domain name as hosted in some account, you normally set a pair of Name Servers to direct it to that specific service provider. On their end, three records are created automatically right after the domain is added - one A record and two MX records. The former is a numeric address, or IP address, which “tells” the Internet domain where its website is, while the other two are alphanumeric and they show the server that deals with the e-mails for that specific domain. The site and the email hosting are typically perceived as one thing, when they're actually two different services. Having separate records for them will allow you to have them with different companies if you would like. As an example, some new provider can have exceptional uptime for your website, 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 e-mails with the latter, you could get the best of both companies. These records are checked when you want to open a website or send an e-mail - either way, the company whose name servers are used for the domain address will 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're going to see the needed site or your email is going to be delivered.