The term “hosting” does not describe a particular service, but a set of services which offer numerous functions to a domain address. Having a site and emails, as an example, are two individual services although in the general case they come together, so a lot of people think of them as one single service. In reality, each and every domain has a couple of DNS records called A and MX, which show the server that deals with each particular service - the first one is a numeric IP address, that defines where the website for the domain is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain. As an example, an A record can be 220.127.116.11 and an MX record can be mx1.domain.com. Every time you open a site or send an email, the global DNS servers are contacted to check the name servers that a Internet domain has and the traffic/message is first forwarded to that company. When you have custom records on their end, the browser request or the email will then be sent to the correct server. The concept behind working with separate records is that the two services use different web protocols and you could have your site hosted by one company and the e-mails by another.