The term “hosting” does not describe one service, but a number of services which provide different functions to a domain. Having a website and emails, for instance, are two individual services even though in the general case they come together, so many people think of them as one single service. The truth is, each domain has a couple of DNS records called A and MX, which show the server that handles each specific service - the first one 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 name. As an example, an A record would be 184.108.40.206 and an MX record would be mx1.domain.com. Each time you open a site or send an email, the global DNS servers are contacted to check the name servers that a domain has and the traffic/message is first forwarded to that company. In case you have custom records on their end, the browser request or the e-mail will be forwarded to the correct server. The reasoning behind employing separate records is that the two services employ different web protocols and you can have your website hosted by one provider and the e-mails by another.