The term “hosting” doesn't describe only one service, but a set of services that provide various functions to a domain. Having a site and emails, for instance, are two individual services even though in the general case they come together, so many people consider them as one single service. In fact, each domain has a several DNS records called A and MX, which show the server that deals with each specific service - the first one is a numeric IP address, which specifies where the site for the domain address is loaded from, while the second one is an alphanumeric string, which shows the server that deals with the emails for the domain name. For instance, an A record can be 220.127.116.11 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 address has and the traffic/message is first forwarded to that company. If you have custom records on their end, the web browser request or the e-mail will be sent to the correct server. The reasoning behind employing separate records is that the two services employ different web protocols and you may have your site hosted by one provider and the emails by another.