The term “hosting” does not describe a single service, but several services which offer a variety of functions to a domain name. Having a site and emails, as an example, are two independent services though in the general case they come together, so most people see them as one single service. Actually, every domain has a several DNS records called A and MX, which show the server that deals with each specific service - the former is a numeric IP address, that defines where the website for the domain address is loaded from, while the latter is an alphanumeric string, which shows the server that deals with the emails for the domain name. For instance, an A record is 188.8.131.52 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 Internet browser request or the e-mail will then be sent to the correct server. The reasoning behind employing separate records is that the two services employ different web protocols and you could have your site hosted by one provider and the e-mail messages by another.