The definition of “hosting” does not describe a particular service, but a set of services which provide a variety of functions to a domain. Having a site and e-mails, as an example, are two individual services despite the fact that in the general case they come together, so most people see them as one single service. In reality, every single domain has a several DNS records called A and MX, which show the server that manages each particular service - the former is a numeric IP address, that defines where the site for the domain is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the e-mails for the domain. For example, an A record is 220.127.116.11 and an MX record can be mx1.domain.com. Each time you open a website 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 directed to that company. When you have custom records on their end, the Internet browser request or the email will then be directed to the correct server. The idea behind employing separate records is that the two services use different web protocols and you may have your site hosted by one service provider and the emails by another.