The word “hosting” doesn't describe just one service, but a set of services that offer numerous functions to a domain name. Having a site and e-mails, for instance, are two independent services despite the fact that in the general case they come together, so most of the people consider them as one single service. In reality, every domain has a number of DNS records called A and MX, which show the server that deals with each specific service - the first one is a numeric IP address, that identifies where the site for the domain name is loaded from, while the second one is an alphanumeric string, which shows the server that handles the e-mails for the domain. For example, an A record is 188.8.131.52 and an MX record can be mx1.domain.com. Whenever you open a site or send an email, the global DNS servers are contacted to check the name servers that a domain name has and the traffic/message is first forwarded to that company. In case you have custom records on their end, the Internet browser request or the e-mail will be sent to the correct server. The reasoning behind employing separate records is that the two services use different web protocols and you could have your website hosted by one service provider and the e-mails by another.